[PATCH 000/141] Fix fall-through warnings for Clang

[PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

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Hi all,

This series aims to fix almost all remaining fall-through warnings in
order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
add multiple break/goto/return/fallthrough statements instead of just
letting the code fall through to the next case.

Notice that in order to enable -Wimplicit-fallthrough for Clang, this
change[1] is meant to be reverted at some point. So, this patch helps
to move in that direction.

Something important to mention is that there is currently a discrepancy
between GCC and Clang when dealing with switch fall-through to empty case
statements or to cases that only contain a break/continue/return
statement[2][3][4].

Now that the -Wimplicit-fallthrough option has been globally enabled[5],
any compiler should really warn on missing either a fallthrough annotation
or any of the other case-terminating statements (break/continue/return/
goto) when falling through to the next case statement. Making exceptions
to this introduces variation in case handling which may continue to lead
to bugs, misunderstandings, and a general lack of robustness. The point
of enabling options like -Wimplicit-fallthrough is to prevent human error
and aid developers in spotting bugs before their code is even built/
submitted/committed, therefore eliminating classes of bugs. So, in order
to really accomplish this, we should, and can, move in the direction of
addressing any error-prone scenarios and get rid of the unintentional
fallthrough bug-class in the kernel, entirely, even if there is some minor
redundancy. Better to have explicit case-ending statements than continue to
have exceptions where one must guess as to the right result. The compiler
will eliminate any actual redundancy.

Note that there is already a patch in mainline that addresses almost
40,000 of these issues[6].

I'm happy to carry this whole series in my own tree if people are OK
with it. :)

[1] commit e2079e93f562c ("kbuild: Do not enable -Wimplicit-fallthrough for clang for now")
[2] ClangBuiltLinux#636
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432
[4] https://godbolt.org/z/xgkvIh
[5] commit a035d552a93b ("Makefile: Globally enable fall-through warning")
[6] commit 4169e889e588 ("include: jhash/signal: Fix fall-through warnings for Clang")

Thanks!

Gustavo A. R. Silva (141):
  afs: Fix fall-through warnings for Clang
  ASoC: codecs: Fix fall-through warnings for Clang
  cifs: Fix fall-through warnings for Clang
  drm/amdgpu: Fix fall-through warnings for Clang
  drm/radeon: Fix fall-through warnings for Clang
  gfs2: Fix fall-through warnings for Clang
  gpio: Fix fall-through warnings for Clang
  IB/hfi1: Fix fall-through warnings for Clang
  igb: Fix fall-through warnings for Clang
  ima: Fix fall-through warnings for Clang
  ipv4: Fix fall-through warnings for Clang
  ixgbe: Fix fall-through warnings for Clang
  media: dvb-frontends: Fix fall-through warnings for Clang
  media: usb: dvb-usb-v2: Fix fall-through warnings for Clang
  netfilter: Fix fall-through warnings for Clang
  nfsd: Fix fall-through warnings for Clang
  nfs: Fix fall-through warnings for Clang
  qed: Fix fall-through warnings for Clang
  qlcnic: Fix fall-through warnings for Clang
  scsi: aic7xxx: Fix fall-through warnings for Clang
  scsi: aic94xx: Fix fall-through warnings for Clang
  scsi: bfa: Fix fall-through warnings for Clang
  staging: rtl8723bs: core: Fix fall-through warnings for Clang
  staging: vt6655: Fix fall-through warnings for Clang
  bnxt_en: Fix fall-through warnings for Clang
  ceph: Fix fall-through warnings for Clang
  drbd: Fix fall-through warnings for Clang
  drm/amd/display: Fix fall-through warnings for Clang
  e1000: Fix fall-through warnings for Clang
  ext2: Fix fall-through warnings for Clang
  ext4: Fix fall-through warnings for Clang
  floppy: Fix fall-through warnings for Clang
  fm10k: Fix fall-through warnings for Clang
  IB/mlx4: Fix fall-through warnings for Clang
  IB/qedr: Fix fall-through warnings for Clang
  ice: Fix fall-through warnings for Clang
  Input: pcspkr - Fix fall-through warnings for Clang
  isofs: Fix fall-through warnings for Clang
  ixgbevf: Fix fall-through warnings for Clang
  kprobes/x86: Fix fall-through warnings for Clang
  mm: Fix fall-through warnings for Clang
  net: 3c509: Fix fall-through warnings for Clang
  net: cassini: Fix fall-through warnings for Clang
  net/mlx4: Fix fall-through warnings for Clang
  net: mscc: ocelot: Fix fall-through warnings for Clang
  netxen_nic: Fix fall-through warnings for Clang
  nfp: Fix fall-through warnings for Clang
  perf/x86: Fix fall-through warnings for Clang
  pinctrl: Fix fall-through warnings for Clang
  RDMA/mlx5: Fix fall-through warnings for Clang
  reiserfs: Fix fall-through warnings for Clang
  security: keys: Fix fall-through warnings for Clang
  selinux: Fix fall-through warnings for Clang
  target: Fix fall-through warnings for Clang
  uprobes/x86: Fix fall-through warnings for Clang
  vxge: Fix fall-through warnings for Clang
  watchdog: Fix fall-through warnings for Clang
  xen-blkfront: Fix fall-through warnings for Clang
  regulator: as3722: Fix fall-through warnings for Clang
  habanalabs: Fix fall-through warnings for Clang
  tee: Fix fall-through warnings for Clang
  HID: usbhid: Fix fall-through warnings for Clang
  HID: input: Fix fall-through warnings for Clang
  ACPI: Fix fall-through warnings for Clang
  airo: Fix fall-through warnings for Clang
  ALSA: hdspm: Fix fall-through warnings for Clang
  ALSA: pcsp: Fix fall-through warnings for Clang
  ALSA: sb: Fix fall-through warnings for Clang
  ath5k: Fix fall-through warnings for Clang
  atm: fore200e: Fix fall-through warnings for Clang
  braille_console: Fix fall-through warnings for Clang
  can: peak_usb: Fix fall-through warnings for Clang
  carl9170: Fix fall-through warnings for Clang
  cfg80211: Fix fall-through warnings for Clang
  crypto: ccree - Fix fall-through warnings for Clang
  decnet: Fix fall-through warnings for Clang
  dm raid: Fix fall-through warnings for Clang
  drm/amd/pm: Fix fall-through warnings for Clang
  drm: Fix fall-through warnings for Clang
  drm/i915/gem: Fix fall-through warnings for Clang
  drm/nouveau/clk: Fix fall-through warnings for Clang
  drm/nouveau: Fix fall-through warnings for Clang
  drm/nouveau/therm: Fix fall-through warnings for Clang
  drm/via: Fix fall-through warnings for Clang
  firewire: core: Fix fall-through warnings for Clang
  hwmon: (corsair-cpro) Fix fall-through warnings for Clang
  hwmon: (max6621) Fix fall-through warnings for Clang
  i3c: master: cdns: Fix fall-through warnings for Clang
  ide: Fix fall-through warnings for Clang
  iio: adc: cpcap: Fix fall-through warnings for Clang
  iwlwifi: iwl-drv: Fix fall-through warnings for Clang
  libata: Fix fall-through warnings for Clang
  mac80211: Fix fall-through warnings for Clang
  media: atomisp: Fix fall-through warnings for Clang
  media: dvb_frontend: Fix fall-through warnings for Clang
  media: rcar_jpu: Fix fall-through warnings for Clang
  media: saa7134: Fix fall-through warnings for Clang
  mmc: sdhci-of-arasan: Fix fall-through warnings for Clang
  mt76: mt7615: Fix fall-through warnings for Clang
  mtd: cfi: Fix fall-through warnings for Clang
  mtd: mtdchar: Fix fall-through warnings for Clang
  mtd: onenand: Fix fall-through warnings for Clang
  mtd: rawnand: fsmc: Fix fall-through warnings for Clang
  mtd: rawnand: stm32_fmc2: Fix fall-through warnings for Clang
  net: ax25: Fix fall-through warnings for Clang
  net: bridge: Fix fall-through warnings for Clang
  net: core: Fix fall-through warnings for Clang
  netfilter: ipt_REJECT: Fix fall-through warnings for Clang
  net: netrom: Fix fall-through warnings for Clang
  net/packet: Fix fall-through warnings for Clang
  net: plip: Fix fall-through warnings for Clang
  net: rose: Fix fall-through warnings for Clang
  nl80211: Fix fall-through warnings for Clang
  phy: qcom-usb-hs: Fix fall-through warnings for Clang
  rds: Fix fall-through warnings for Clang
  rt2x00: Fix fall-through warnings for Clang
  rtl8xxxu: Fix fall-through warnings for Clang
  rtw88: Fix fall-through warnings for Clang
  rxrpc: Fix fall-through warnings for Clang
  scsi: aacraid: Fix fall-through warnings for Clang
  scsi: aha1740: Fix fall-through warnings for Clang
  scsi: csiostor: Fix fall-through warnings for Clang
  scsi: lpfc: Fix fall-through warnings for Clang
  scsi: stex: Fix fall-through warnings for Clang
  sctp: Fix fall-through warnings for Clang
  slimbus: messaging: Fix fall-through warnings for Clang
  staging: qlge: Fix fall-through warnings for Clang
  staging: vt6656: Fix fall-through warnings for Clang
  SUNRPC: Fix fall-through warnings for Clang
  tipc: Fix fall-through warnings for Clang
  tpm: Fix fall-through warnings for Clang
  ubi: Fix fall-through warnings for Clang
  usb: Fix fall-through warnings for Clang
  video: fbdev: lxfb_ops: Fix fall-through warnings for Clang
  video: fbdev: pm2fb: Fix fall-through warnings for Clang
  virtio_net: Fix fall-through warnings for Clang
  wcn36xx: Fix fall-through warnings for Clang
  xen/manage: Fix fall-through warnings for Clang
  xfrm: Fix fall-through warnings for Clang
  zd1201: Fix fall-through warnings for Clang
  Input: libps2 - Fix fall-through warnings for Clang

 arch/x86/events/core.c                                    | 2 +-
 arch/x86/kernel/kprobes/core.c                            | 1 +
 arch/x86/kernel/uprobes.c                                 | 2 ++
 drivers/accessibility/braille/braille_console.c           | 1 +
 drivers/acpi/sbshc.c                                      | 1 +
 drivers/ata/libata-eh.c                                   | 1 +
 drivers/atm/fore200e.c                                    | 1 +
 drivers/block/drbd/drbd_receiver.c                        | 1 +
 drivers/block/drbd/drbd_req.c                             | 1 +
 drivers/block/floppy.c                                    | 1 +
 drivers/block/xen-blkfront.c                              | 1 +
 drivers/char/tpm/eventlog/tpm1.c                          | 1 +
 drivers/crypto/ccree/cc_cipher.c                          | 3 +++
 drivers/firewire/core-topology.c                          | 1 +
 drivers/gpio/gpio-ath79.c                                 | 1 +
 drivers/gpio/gpiolib-acpi.c                               | 1 +
 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c                    | 1 +
 drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c                     | 1 +
 drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c                     | 1 +
 drivers/gpu/drm/amd/amdgpu/vi.c                           | 1 +
 drivers/gpu/drm/amd/display/dc/bios/bios_parser.c         | 1 +
 drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c        | 2 ++
 drivers/gpu/drm/amd/display/dc/core/dc_link.c             | 1 +
 drivers/gpu/drm/amd/pm/powerplay/si_dpm.c                 | 2 +-
 .../gpu/drm/amd/pm/powerplay/smumgr/polaris10_smumgr.c    | 1 +
 drivers/gpu/drm/drm_bufs.c                                | 1 +
 drivers/gpu/drm/i915/gem/i915_gem_shrinker.c              | 1 +
 drivers/gpu/drm/nouveau/nouveau_bo.c                      | 1 +
 drivers/gpu/drm/nouveau/nouveau_connector.c               | 1 +
 drivers/gpu/drm/nouveau/nvkm/subdev/clk/nv50.c            | 1 +
 drivers/gpu/drm/nouveau/nvkm/subdev/therm/gf119.c         | 1 +
 drivers/gpu/drm/radeon/ci_dpm.c                           | 2 +-
 drivers/gpu/drm/radeon/r300.c                             | 1 +
 drivers/gpu/drm/radeon/si_dpm.c                           | 2 +-
 drivers/gpu/drm/via/via_irq.c                             | 1 +
 drivers/hid/hid-input.c                                   | 1 +
 drivers/hid/usbhid/hid-core.c                             | 2 ++
 drivers/hwmon/corsair-cpro.c                              | 1 +
 drivers/hwmon/max6621.c                                   | 2 +-
 drivers/i3c/master/i3c-master-cdns.c                      | 2 ++
 drivers/ide/siimage.c                                     | 1 +
 drivers/iio/adc/cpcap-adc.c                               | 1 +
 drivers/infiniband/hw/hfi1/qp.c                           | 1 +
 drivers/infiniband/hw/hfi1/tid_rdma.c                     | 5 +++++
 drivers/infiniband/hw/mlx4/mad.c                          | 1 +
 drivers/infiniband/hw/mlx5/qp.c                           | 1 +
 drivers/infiniband/hw/qedr/main.c                         | 1 +
 drivers/input/misc/pcspkr.c                               | 1 +
 drivers/input/serio/libps2.c                              | 2 +-
 drivers/md/dm-raid.c                                      | 1 +
 drivers/media/dvb-core/dvb_frontend.c                     | 1 +
 drivers/media/dvb-frontends/cx24120.c                     | 1 +
 drivers/media/dvb-frontends/dib0090.c                     | 2 ++
 drivers/media/dvb-frontends/drxk_hard.c                   | 1 +
 drivers/media/dvb-frontends/m88rs2000.c                   | 1 +
 drivers/media/pci/saa7134/saa7134-tvaudio.c               | 1 +
 drivers/media/platform/rcar_jpu.c                         | 1 +
 drivers/media/usb/dvb-usb-v2/af9015.c                     | 1 +
 drivers/media/usb/dvb-usb-v2/lmedm04.c                    | 1 +
 drivers/misc/habanalabs/gaudi/gaudi.c                     | 1 +
 drivers/mmc/host/sdhci-of-arasan.c                        | 4 ++++
 drivers/mtd/chips/cfi_cmdset_0001.c                       | 1 +
 drivers/mtd/chips/cfi_cmdset_0002.c                       | 2 ++
 drivers/mtd/chips/cfi_cmdset_0020.c                       | 2 ++
 drivers/mtd/mtdchar.c                                     | 1 +
 drivers/mtd/nand/onenand/onenand_samsung.c                | 1 +
 drivers/mtd/nand/raw/fsmc_nand.c                          | 1 +
 drivers/mtd/nand/raw/stm32_fmc2_nand.c                    | 2 ++
 drivers/mtd/ubi/build.c                                   | 1 +
 drivers/net/can/usb/peak_usb/pcan_usb_core.c              | 2 ++
 drivers/net/ethernet/3com/3c509.c                         | 1 +
 drivers/net/ethernet/broadcom/bnxt/bnxt.c                 | 1 +
 drivers/net/ethernet/intel/e1000/e1000_hw.c               | 1 +
 drivers/net/ethernet/intel/fm10k/fm10k_mbx.c              | 2 ++
 drivers/net/ethernet/intel/ice/ice_txrx_lib.c             | 1 +
 drivers/net/ethernet/intel/igb/e1000_phy.c                | 1 +
 drivers/net/ethernet/intel/igb/igb_ethtool.c              | 1 +
 drivers/net/ethernet/intel/igb/igb_ptp.c                  | 1 +
 drivers/net/ethernet/intel/ixgbe/ixgbe_82599.c            | 2 ++
 drivers/net/ethernet/intel/ixgbe/ixgbe_common.c           | 1 +
 drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c              | 1 +
 drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c              | 1 +
 drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c         | 1 +
 drivers/net/ethernet/mellanox/mlx4/resource_tracker.c     | 1 +
 drivers/net/ethernet/mscc/ocelot_vcap.c                   | 1 +
 drivers/net/ethernet/neterion/vxge/vxge-config.c          | 1 +
 drivers/net/ethernet/netronome/nfp/nfp_net_repr.c         | 1 +
 drivers/net/ethernet/qlogic/netxen/netxen_nic_init.c      | 1 +
 drivers/net/ethernet/qlogic/qed/qed_l2.c                  | 1 +
 drivers/net/ethernet/qlogic/qed/qed_sriov.c               | 1 +
 drivers/net/ethernet/qlogic/qlcnic/qlcnic_io.c            | 1 +
 drivers/net/ethernet/qlogic/qlcnic/qlcnic_main.c          | 1 +
 drivers/net/ethernet/sun/cassini.c                        | 1 +
 drivers/net/plip/plip.c                                   | 2 ++
 drivers/net/virtio_net.c                                  | 1 +
 drivers/net/wireless/ath/ath5k/mac80211-ops.c             | 1 +
 drivers/net/wireless/ath/carl9170/tx.c                    | 1 +
 drivers/net/wireless/ath/wcn36xx/smd.c                    | 2 +-
 drivers/net/wireless/cisco/airo.c                         | 1 +
 drivers/net/wireless/intel/iwlwifi/iwl-drv.c              | 2 +-
 drivers/net/wireless/mediatek/mt76/mt7615/eeprom.c        | 2 +-
 drivers/net/wireless/ralink/rt2x00/rt2x00queue.c          | 1 +
 drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c     | 8 ++++----
 drivers/net/wireless/realtek/rtw88/fw.c                   | 2 +-
 drivers/net/wireless/zydas/zd1201.c                       | 2 +-
 drivers/phy/qualcomm/phy-qcom-usb-hs.c                    | 1 +
 drivers/pinctrl/renesas/pinctrl-rza1.c                    | 1 +
 drivers/regulator/as3722-regulator.c                      | 3 ++-
 drivers/scsi/aacraid/commsup.c                            | 1 +
 drivers/scsi/aha1740.c                                    | 1 +
 drivers/scsi/aic7xxx/aic79xx_core.c                       | 4 +++-
 drivers/scsi/aic7xxx/aic7xxx_core.c                       | 4 ++--
 drivers/scsi/aic94xx/aic94xx_scb.c                        | 2 ++
 drivers/scsi/aic94xx/aic94xx_task.c                       | 2 ++
 drivers/scsi/bfa/bfa_fcs_lport.c                          | 2 +-
 drivers/scsi/bfa/bfa_ioc.c                                | 6 ++++--
 drivers/scsi/csiostor/csio_wr.c                           | 1 +
 drivers/scsi/lpfc/lpfc_bsg.c                              | 1 +
 drivers/scsi/stex.c                                       | 1 +
 drivers/slimbus/messaging.c                               | 1 +
 drivers/staging/media/atomisp/pci/runtime/isys/src/rx.c   | 1 +
 drivers/staging/qlge/qlge_main.c                          | 1 +
 drivers/staging/rtl8723bs/core/rtw_cmd.c                  | 1 +
 drivers/staging/rtl8723bs/core/rtw_mlme_ext.c             | 1 +
 drivers/staging/rtl8723bs/core/rtw_wlan_util.c            | 1 +
 drivers/staging/vt6655/device_main.c                      | 1 +
 drivers/staging/vt6655/rxtx.c                             | 2 ++
 drivers/staging/vt6656/main_usb.c                         | 1 +
 drivers/target/target_core_iblock.c                       | 1 +
 drivers/target/target_core_pr.c                           | 1 +
 drivers/tee/tee_core.c                                    | 1 +
 drivers/usb/gadget/function/f_fs.c                        | 2 ++
 drivers/usb/gadget/function/f_loopback.c                  | 2 +-
 drivers/usb/gadget/function/f_sourcesink.c                | 1 +
 drivers/usb/gadget/udc/dummy_hcd.c                        | 2 ++
 drivers/usb/host/fotg210-hcd.c                            | 2 +-
 drivers/usb/host/isp116x-hcd.c                            | 1 +
 drivers/usb/host/max3421-hcd.c                            | 1 +
 drivers/usb/host/oxu210hp-hcd.c                           | 1 +
 drivers/usb/misc/yurex.c                                  | 1 +
 drivers/usb/musb/tusb6010.c                               | 1 +
 drivers/usb/storage/ene_ub6250.c                          | 1 +
 drivers/usb/storage/uas.c                                 | 1 +
 drivers/video/fbdev/geode/lxfb_ops.c                      | 1 +
 drivers/video/fbdev/pm2fb.c                               | 1 +
 drivers/watchdog/machzwd.c                                | 1 +
 drivers/xen/manage.c                                      | 1 +
 fs/afs/cmservice.c                                        | 5 +++++
 fs/afs/fsclient.c                                         | 4 ++++
 fs/afs/vlclient.c                                         | 1 +
 fs/ceph/dir.c                                             | 2 ++
 fs/cifs/inode.c                                           | 1 +
 fs/cifs/sess.c                                            | 1 +
 fs/cifs/smbdirect.c                                       | 1 +
 fs/ext2/inode.c                                           | 1 +
 fs/ext4/super.c                                           | 1 +
 fs/gfs2/inode.c                                           | 2 ++
 fs/gfs2/recovery.c                                        | 1 +
 fs/isofs/rock.c                                           | 1 +
 fs/nfs/nfs3acl.c                                          | 1 +
 fs/nfs/nfs4client.c                                       | 1 +
 fs/nfs/nfs4proc.c                                         | 2 ++
 fs/nfs/nfs4state.c                                        | 1 +
 fs/nfs/pnfs.c                                             | 2 ++
 fs/nfsd/nfs4state.c                                       | 1 +
 fs/nfsd/nfsctl.c                                          | 1 +
 fs/reiserfs/namei.c                                       | 1 +
 mm/mm_init.c                                              | 1 +
 mm/vmscan.c                                               | 1 +
 net/ax25/af_ax25.c                                        | 1 +
 net/bridge/br_input.c                                     | 1 +
 net/core/dev.c                                            | 1 +
 net/decnet/dn_route.c                                     | 2 +-
 net/ipv4/ah4.c                                            | 1 +
 net/ipv4/esp4.c                                           | 1 +
 net/ipv4/fib_semantics.c                                  | 1 +
 net/ipv4/ip_vti.c                                         | 1 +
 net/ipv4/ipcomp.c                                         | 1 +
 net/ipv4/netfilter/ipt_REJECT.c                           | 1 +
 net/mac80211/cfg.c                                        | 2 ++
 net/netfilter/nf_conntrack_proto_dccp.c                   | 1 +
 net/netfilter/nf_tables_api.c                             | 1 +
 net/netfilter/nft_ct.c                                    | 1 +
 net/netrom/nr_route.c                                     | 4 ++++
 net/packet/af_packet.c                                    | 1 +
 net/rds/tcp_connect.c                                     | 1 +
 net/rds/threads.c                                         | 2 ++
 net/rose/rose_route.c                                     | 2 ++
 net/rxrpc/af_rxrpc.c                                      | 1 +
 net/sctp/input.c                                          | 3 ++-
 net/sunrpc/rpc_pipe.c                                     | 1 +
 net/sunrpc/xprtsock.c                                     | 1 +
 net/tipc/link.c                                           | 1 +
 net/wireless/nl80211.c                                    | 1 +
 net/wireless/util.c                                       | 1 +
 net/xfrm/xfrm_interface.c                                 | 1 +
 security/integrity/ima/ima_main.c                         | 1 +
 security/integrity/ima/ima_policy.c                       | 2 ++
 security/keys/process_keys.c                              | 1 +
 security/selinux/hooks.c                                  | 1 +
 sound/drivers/pcsp/pcsp_input.c                           | 1 +
 sound/isa/sb/sb8_main.c                                   | 1 +
 sound/pci/rme9652/hdspm.c                                 | 1 +
 sound/soc/codecs/adav80x.c                                | 1 +
 sound/soc/codecs/arizona.c                                | 1 +
 sound/soc/codecs/cs42l52.c                                | 1 +
 sound/soc/codecs/cs42l56.c                                | 1 +
 sound/soc/codecs/cs47l92.c                                | 1 +
 sound/soc/codecs/wm8962.c                                 | 1 +
 209 files changed, 264 insertions(+), 26 deletions(-)

-- 
2.27.0

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

[-- Attachment #1: Type: text/plain, Size: 671 bytes --]

On Fri, 2020-11-20 at 12:21 -0600, Gustavo A. R. Silva wrote:
> Hi all,
> 
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> change[1] is meant to be reverted at some point. So, this patch helps
> to move in that direction.

This was a bit hard to parse for a second or three.

Thanks Gustavo.

How was this change done?

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Jakub Kicinski]

[-- Attachment #1: Type: text/plain, Size: 1082 bytes --]

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> change[1] is meant to be reverted at some point. So, this patch helps
> to move in that direction.
> 
> Something important to mention is that there is currently a discrepancy
> between GCC and Clang when dealing with switch fall-through to empty case
> statements or to cases that only contain a break/continue/return
> statement[2][3][4].

Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped
find?

IMVHO compiler warnings are supposed to warn about issues that could
be bugs. Falling through to default: break; can hardly be a bug?!

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

[-- Attachment #1: Type: text/plain, Size: 948 bytes --]



On 11/20/20 12:28, Joe Perches wrote:
> On Fri, 2020-11-20 at 12:21 -0600, Gustavo A. R. Silva wrote:
>> Hi all,
>>
>> This series aims to fix almost all remaining fall-through warnings in
>> order to enable -Wimplicit-fallthrough for Clang.
>>
>> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
>> add multiple break/goto/return/fallthrough statements instead of just
>> letting the code fall through to the next case.
>>
>> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
>> change[1] is meant to be reverted at some point. So, this patch helps
>> to move in that direction.
> 
> This was a bit hard to parse for a second or three.
> 
> Thanks Gustavo.
> 
> How was this change done?

I audited case by case in order to determine the best fit for each
situation. Depending on the surrounding logic, sometimes it makes
more sense a goto or a fallthrough rather than merely a break.

Thanks
--
Gustavo

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

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Hi,

On 11/20/20 12:53, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:
>> This series aims to fix almost all remaining fall-through warnings in
>> order to enable -Wimplicit-fallthrough for Clang.
>>
>> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
>> add multiple break/goto/return/fallthrough statements instead of just
>> letting the code fall through to the next case.
>>
>> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
>> change[1] is meant to be reverted at some point. So, this patch helps
>> to move in that direction.
>>
>> Something important to mention is that there is currently a discrepancy
>> between GCC and Clang when dealing with switch fall-through to empty case
>> statements or to cases that only contain a break/continue/return
>> statement[2][3][4].
> 
> Are we sure we want to make this change? Was it discussed before?
> 
> Are there any bugs Clangs puritanical definition of fallthrough helped
> find?
> 
> IMVHO compiler warnings are supposed to warn about issues that could
> be bugs. Falling through to default: break; can hardly be a bug?!

The justification for this is explained in this same changelog text:

Now that the -Wimplicit-fallthrough option has been globally enabled[5],
any compiler should really warn on missing either a fallthrough annotation
or any of the other case-terminating statements (break/continue/return/
goto) when falling through to the next case statement. Making exceptions
to this introduces variation in case handling which may continue to lead
to bugs, misunderstandings, and a general lack of robustness. The point
of enabling options like -Wimplicit-fallthrough is to prevent human error
and aid developers in spotting bugs before their code is even built/
submitted/committed, therefore eliminating classes of bugs. So, in order
to really accomplish this, we should, and can, move in the direction of
addressing any error-prone scenarios and get rid of the unintentional
fallthrough bug-class in the kernel, entirely, even if there is some minor
redundancy. Better to have explicit case-ending statements than continue to
have exceptions where one must guess as to the right result. The compiler
will eliminate any actual redundancy.

Note that there is already a patch in mainline that addresses almost
40,000 of these issues[6].

[1] commit e2079e93f562c ("kbuild: Do not enable -Wimplicit-fallthrough for clang for now")
[2] ClangBuiltLinux#636
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432
[4] https://godbolt.org/z/xgkvIh
[5] commit a035d552a93b ("Makefile: Globally enable fall-through warning")
[6] commit 4169e889e588 ("include: jhash/signal: Fix fall-through warnings for Clang")

Thanks
--
Gustavo

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

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On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:
> > This series aims to fix almost all remaining fall-through warnings in
> > order to enable -Wimplicit-fallthrough for Clang.
> > 
> > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > add multiple break/goto/return/fallthrough statements instead of just
> > letting the code fall through to the next case.
> > 
> > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > change[1] is meant to be reverted at some point. So, this patch helps
> > to move in that direction.
> > 
> > Something important to mention is that there is currently a discrepancy
> > between GCC and Clang when dealing with switch fall-through to empty case
> > statements or to cases that only contain a break/continue/return
> > statement[2][3][4].
> 
> Are we sure we want to make this change? Was it discussed before?
> 
> Are there any bugs Clangs puritanical definition of fallthrough helped
> find?
> 
> IMVHO compiler warnings are supposed to warn about issues that could
> be bugs. Falling through to default: break; can hardly be a bug?!

It's certainly a place where the intent is not always clear. I think
this makes all the cases unambiguous, and doesn't impact the machine
code, since the compiler will happily optimize away any behavioral
redundancy.


-- 
Kees Cook

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Jakub Kicinski]

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On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > This series aims to fix almost all remaining fall-through warnings in
> > > order to enable -Wimplicit-fallthrough for Clang.
> > > 
> > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > add multiple break/goto/return/fallthrough statements instead of just
> > > letting the code fall through to the next case.
> > > 
> > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > change[1] is meant to be reverted at some point. So, this patch helps
> > > to move in that direction.
> > > 
> > > Something important to mention is that there is currently a discrepancy
> > > between GCC and Clang when dealing with switch fall-through to empty case
> > > statements or to cases that only contain a break/continue/return
> > > statement[2][3][4].  
> > 
> > Are we sure we want to make this change? Was it discussed before?
> > 
> > Are there any bugs Clangs puritanical definition of fallthrough helped
> > find?
> > 
> > IMVHO compiler warnings are supposed to warn about issues that could
> > be bugs. Falling through to default: break; can hardly be a bug?!  
> 
> It's certainly a place where the intent is not always clear. I think
> this makes all the cases unambiguous, and doesn't impact the machine
> code, since the compiler will happily optimize away any behavioral
> redundancy.

If none of the 140 patches here fix a real bug, and there is no change
to machine code then it sounds to me like a W=2 kind of a warning.

I think clang is just being annoying here, but if I'm the only one who
feels this way chances are I'm wrong :)

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

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On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > This series aims to fix almost all remaining fall-through warnings in
> > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > 
> > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > letting the code fall through to the next case.
> > > > 
> > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > to move in that direction.
> > > > 
> > > > Something important to mention is that there is currently a discrepancy
> > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > statements or to cases that only contain a break/continue/return
> > > > statement[2][3][4].  
> > > 
> > > Are we sure we want to make this change? Was it discussed before?
> > > 
> > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > find?
> > > 
> > > IMVHO compiler warnings are supposed to warn about issues that could
> > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > 
> > It's certainly a place where the intent is not always clear. I think
> > this makes all the cases unambiguous, and doesn't impact the machine
> > code, since the compiler will happily optimize away any behavioral
> > redundancy.
> 
> If none of the 140 patches here fix a real bug, and there is no change
> to machine code then it sounds to me like a W=2 kind of a warning.

I'd like to avoid splitting common -W options between default and W=2
just based on the compiler. Getting -Wimplicit-fallthrough enabled found
plenty of bugs, so making sure it works correctly for both compilers
feels justified to me. (This is just a subset of the same C language
short-coming.)

> I think clang is just being annoying here, but if I'm the only one who
> feels this way chances are I'm wrong :)

It's being pretty pedantic, but I don't think it's unreasonable to
explicitly state how every case ends. GCC's silence for the case of
"fall through to a break" doesn't really seem justified.

-- 
Kees Cook

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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Hi Gustavo,

On Fri, Nov 20, 2020 at 7:21 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
>
> Hi all,
>
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.

Thanks for this.

Since this warning is reliable in both/all compilers and we are
eventually getting rid of all the cases, what about going even further
and making it an error right after?

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

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On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > This series aims to fix almost all remaining fall-through warnings in
> > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > 
> > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > letting the code fall through to the next case.
> > > > 
> > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > to move in that direction.
> > > > 
> > > > Something important to mention is that there is currently a discrepancy
> > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > statements or to cases that only contain a break/continue/return
> > > > statement[2][3][4].  
> > > 
> > > Are we sure we want to make this change? Was it discussed before?
> > > 
> > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > find?
> > > 
> > > IMVHO compiler warnings are supposed to warn about issues that could
> > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > 
> > It's certainly a place where the intent is not always clear. I think
> > this makes all the cases unambiguous, and doesn't impact the machine
> > code, since the compiler will happily optimize away any behavioral
> > redundancy.
> 
> If none of the 140 patches here fix a real bug, and there is no change
> to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far:
https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/

-- 
Kees Cook

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Sun, 2020-11-22 at 08:17 -0800, Kees Cook wrote:
> On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > > This series aims to fix almost all remaining fall-through
> > > > > warnings in order to enable -Wimplicit-fallthrough for Clang.
> > > > > 
> > > > > In preparation to enable -Wimplicit-fallthrough for Clang,
> > > > > explicitly add multiple break/goto/return/fallthrough
> > > > > statements instead of just letting the code fall through to
> > > > > the next case.
> > > > > 
> > > > > Notice that in order to enable -Wimplicit-fallthrough for
> > > > > Clang, this change[1] is meant to be reverted at some point.
> > > > > So, this patch helps to move in that direction.
> > > > > 
> > > > > Something important to mention is that there is currently a
> > > > > discrepancy between GCC and Clang when dealing with switch
> > > > > fall-through to empty case statements or to cases that only
> > > > > contain a break/continue/return statement[2][3][4].  
> > > > 
> > > > Are we sure we want to make this change? Was it discussed
> > > > before?
> > > > 
> > > > Are there any bugs Clangs puritanical definition of fallthrough
> > > > helped find?
> > > > 
> > > > IMVHO compiler warnings are supposed to warn about issues that
> > > > could be bugs. Falling through to default: break; can hardly be
> > > > a bug?!  
> > > 
> > > It's certainly a place where the intent is not always clear. I
> > > think this makes all the cases unambiguous, and doesn't impact
> > > the machine code, since the compiler will happily optimize away
> > > any behavioral redundancy.
> > 
> > If none of the 140 patches here fix a real bug, and there is no
> > change to machine code then it sounds to me like a W=2 kind of a
> > warning.
> 
> FWIW, this series has found at least one bug so far:
> https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/


Well, it's a problem in an error leg, sure, but it's not a really
compelling reason for a 141 patch series, is it?  All that fixing this
error will do is get the driver to print "oh dear there's a problem"
under four more conditions than it previously did.

We've been at this for three years now with nearly a thousand patches,
firstly marking all the fall throughs with /* fall through */ and later
changing it to fallthrough.  At some point we do have to ask if the
effort is commensurate with the protection afforded.  Please tell me
our reward for all this effort isn't a single missing error print.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

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On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> Please tell me
> our reward for all this effort isn't a single missing error print.

There were quite literally dozens of logical defects found
by the fallthrough additions.  Very few were logging only.

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:
> On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> > Please tell me our reward for all this effort isn't a single
> > missing error print.
> 
> There were quite literally dozens of logical defects found
> by the fallthrough additions.  Very few were logging only.

So can you give us the best examples (or indeed all of them if someone
is keeping score)?  hopefully this isn't a US election situation ...

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

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On Sun, 2020-11-22 at 11:12 -0800, James Bottomley wrote:
> On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:
> > On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> > > Please tell me our reward for all this effort isn't a single
> > > missing error print.
> > 
> > There were quite literally dozens of logical defects found
> > by the fallthrough additions.  Very few were logging only.
> 
> So can you give us the best examples (or indeed all of them if someone
> is keeping score)?  hopefully this isn't a US election situation ...

Gustavo?  Are you running for congress now?

https://lwn.net/Articles/794944/

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Sun, 2020-11-22 at 11:22 -0800, Joe Perches wrote:
> On Sun, 2020-11-22 at 11:12 -0800, James Bottomley wrote:
> > On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:
> > > On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> > > > Please tell me our reward for all this effort isn't a single
> > > > missing error print.
> > > 
> > > There were quite literally dozens of logical defects found
> > > by the fallthrough additions.  Very few were logging only.
> > 
> > So can you give us the best examples (or indeed all of them if
> > someone is keeping score)?  hopefully this isn't a US election
> > situation ...
> 
> Gustavo?  Are you running for congress now?
> 
> https://lwn.net/Articles/794944/

That's 21 reported fixes of which about 50% seem to produce no change
in code behaviour at all, a quarter seem to have no user visible effect
with the remaining quarter producing unexpected errors on obscure
configuration parameters, which is why no-one really noticed them
before.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Sun, Nov 22, 2020 at 7:22 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, it's a problem in an error leg, sure, but it's not a really
> compelling reason for a 141 patch series, is it?  All that fixing this
> error will do is get the driver to print "oh dear there's a problem"
> under four more conditions than it previously did.
>
> We've been at this for three years now with nearly a thousand patches,
> firstly marking all the fall throughs with /* fall through */ and later
> changing it to fallthrough.  At some point we do have to ask if the
> effort is commensurate with the protection afforded.  Please tell me
> our reward for all this effort isn't a single missing error print.

It isn't that much effort, isn't it? Plus we need to take into account
the future mistakes that it might prevent, too. So even if there were
zero problems found so far, it is still a positive change.

I would agree if these changes were high risk, though; but they are
almost trivial.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Sam Ravnborg]

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Hi James.

> > > If none of the 140 patches here fix a real bug, and there is no
> > > change to machine code then it sounds to me like a W=2 kind of a
> > > warning.
> > 
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/
> 
> 
> Well, it's a problem in an error leg, sure, but it's not a really
> compelling reason for a 141 patch series, is it?  All that fixing this
> error will do is get the driver to print "oh dear there's a problem"
> under four more conditions than it previously did.

You are asking the wrong question here.

Yuo should ask  how many hours could have been saved by all the bugs
people have been fighting with and then fixed *before* the code
hit the kernel at all.

My personal experience is that I, more than once, have had errors
related to a missing break in my code. So this warnings is IMO a win.

And if we are only ~100 patches to have it globally enabled then it is a
no-brainer in my book.

	Sam

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Sun, 2020-11-22 at 21:35 +0100, Miguel Ojeda wrote:
> On Sun, Nov 22, 2020 at 7:22 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > Well, it's a problem in an error leg, sure, but it's not a really
> > compelling reason for a 141 patch series, is it?  All that fixing
> > this error will do is get the driver to print "oh dear there's a
> > problem" under four more conditions than it previously did.
> > 
> > We've been at this for three years now with nearly a thousand
> > patches, firstly marking all the fall throughs with /* fall through
> > */ and later changing it to fallthrough.  At some point we do have
> > to ask if the effort is commensurate with the protection
> > afforded.  Please tell me our reward for all this effort isn't a
> > single missing error print.
> 
> It isn't that much effort, isn't it?

Well, it seems to be three years of someone's time plus the maintainer
review time and series disruption of nearly a thousand patches.  Let's
be conservative and assume the producer worked about 30% on the series
and it takes about 5-10 minutes per patch to review, merge and for
others to rework existing series.  So let's say it's cost a person year
of a relatively junior engineer producing the patches and say 100h of
review and application time.  The latter is likely the big ticket item
because it's what we have in least supply in the kernel (even though
it's 20x vs the producer time).

>  Plus we need to take into account the future mistakes that it might
> prevent, too. So even if there were zero problems found so far, it is
> still a positive change.

Well, the question I was asking is if it's worth the cost which I've
tried to outline above.

> I would agree if these changes were high risk, though; but they are
> almost trivial.

It's not about the risk of the changes it's about the cost of
implementing them.  Even if you discount the producer time (which
someone gets to pay for, and if I were the engineering manager, I'd be
unhappy about), the review/merge/rework time is pretty significant in
exchange for six minor bug fixes.  Fine, when a new compiler warning
comes along it's certainly reasonable to see if we can benefit from it
and the fact that the compiler people think it's worthwhile is enough
evidence to assume this initially.  But at some point you have to ask
whether that assumption is supported by the evidence we've accumulated
over the time we've been using it.  And if the evidence doesn't support
it perhaps it is time to stop the experiment.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Finn Thain]

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On Sun, 22 Nov 2020, Miguel Ojeda wrote:

> 
> It isn't that much effort, isn't it? Plus we need to take into account 
> the future mistakes that it might prevent, too.

We should also take into account optimisim about future improvements in 
tooling.

> So even if there were zero problems found so far, it is still a positive 
> change.
> 

It is if you want to spin it that way.

> I would agree if these changes were high risk, though; but they are 
> almost trivial.
> 

This is trivial:

 case 1:
	this();
+	fallthrough;
 case 2:
 	that();

But what we inevitably get is changes like this:

 case 3:
        this();
+       break;
 case 4:
        hmmm();

Why? Mainly to silence the compiler. Also because the patch author argued 
successfully that they had found a theoretical bug, often in mature code.

But is anyone keeping score of the regressions? If unreported bugs count, 
what about unreported regressions?

> Cheers,
> Miguel
> 

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Mon, 2020-11-23 at 09:54 +1100, Finn Thain wrote:
> But is anyone keeping score of the regressions? If unreported bugs
> count, what about unreported regressions?

Well, I was curious about the former (obviously no tool will tell me
about the latter), so I asked git what patches had a fall-through
series named in a fixes tag and these three popped out:

9cf51446e686 bpf, powerpc: Fix misuse of fallthrough in bpf_jit_comp()
6a9dc5fd6170 lib: Revert use of fallthrough pseudo-keyword in lib/
91dbd73a1739 mips/oprofile: Fix fallthrough placement

I don't think any of these is fixing a significant problem, but they
did cause someone time and trouble to investigate.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Sun, Nov 22, 2020 at 11:54 PM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> We should also take into account optimisim about future improvements in
> tooling.

Not sure what you mean here. There is no reliable way to guess what
the intention was with a missing fallthrough, even if you parsed
whitespace and indentation.

> It is if you want to spin it that way.

How is that a "spin"? It is a fact that we won't get *implicit*
fallthrough mistakes anymore (in particular if we make it a hard
error).

> But what we inevitably get is changes like this:
>
>  case 3:
>         this();
> +       break;
>  case 4:
>         hmmm();
>
> Why? Mainly to silence the compiler. Also because the patch author argued
> successfully that they had found a theoretical bug, often in mature code.

If someone changes control flow, that is on them. Every kernel
developer knows what `break` does.

> But is anyone keeping score of the regressions? If unreported bugs count,
> what about unreported regressions?

Introducing `fallthrough` does not change semantics. If you are really
keen, you can always compare the objects because the generated code
shouldn't change.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, it seems to be three years of someone's time plus the maintainer
> review time and series disruption of nearly a thousand patches.  Let's
> be conservative and assume the producer worked about 30% on the series
> and it takes about 5-10 minutes per patch to review, merge and for
> others to rework existing series.  So let's say it's cost a person year
> of a relatively junior engineer producing the patches and say 100h of
> review and application time.  The latter is likely the big ticket item
> because it's what we have in least supply in the kernel (even though
> it's 20x vs the producer time).

How are you arriving at such numbers? It is a total of ~200 trivial lines.

> It's not about the risk of the changes it's about the cost of
> implementing them.  Even if you discount the producer time (which
> someone gets to pay for, and if I were the engineering manager, I'd be
> unhappy about), the review/merge/rework time is pretty significant in
> exchange for six minor bug fixes.  Fine, when a new compiler warning
> comes along it's certainly reasonable to see if we can benefit from it
> and the fact that the compiler people think it's worthwhile is enough
> evidence to assume this initially.  But at some point you have to ask
> whether that assumption is supported by the evidence we've accumulated
> over the time we've been using it.  And if the evidence doesn't support
> it perhaps it is time to stop the experiment.

Maintainers routinely review 1-line trivial patches, not to mention
internal API changes, etc.

If some company does not want to pay for that, that's fine, but they
don't get to be maintainers and claim `Supported`.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Mon, 2020-11-23 at 15:19 +0100, Miguel Ojeda wrote:
> On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > Well, it seems to be three years of someone's time plus the
> > maintainer review time and series disruption of nearly a thousand
> > patches.  Let's be conservative and assume the producer worked
> > about 30% on the series and it takes about 5-10 minutes per patch
> > to review, merge and for others to rework existing series.  So
> > let's say it's cost a person year of a relatively junior engineer
> > producing the patches and say 100h of review and application
> > time.  The latter is likely the big ticket item because it's what
> > we have in least supply in the kernel (even though it's 20x vs the
> > producer time).
> 
> How are you arriving at such numbers? It is a total of ~200 trivial
> lines.

Well, I used git.  It says that as of today in Linus' tree we have 889
patches related to fall throughs and the first series went in in
october 2017 ... ignoring a couple of outliers back to February.

> > It's not about the risk of the changes it's about the cost of
> > implementing them.  Even if you discount the producer time (which
> > someone gets to pay for, and if I were the engineering manager, I'd
> > be unhappy about), the review/merge/rework time is pretty
> > significant in exchange for six minor bug fixes.  Fine, when a new
> > compiler warning comes along it's certainly reasonable to see if we
> > can benefit from it and the fact that the compiler people think
> > it's worthwhile is enough evidence to assume this initially.  But
> > at some point you have to ask whether that assumption is supported
> > by the evidence we've accumulated over the time we've been using
> > it.  And if the evidence doesn't support it perhaps it is time to
> > stop the experiment.
> 
> Maintainers routinely review 1-line trivial patches, not to mention
> internal API changes, etc.

We're also complaining about the inability to recruit maintainers:

https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/

And burn out:

http://antirez.com/news/129

The whole crux of your argument seems to be maintainers' time isn't
important so we should accept all trivial patches ... I'm pushing back
on that assumption in two places, firstly the valulessness of the time
and secondly that all trivial patches are valuable.

> If some company does not want to pay for that, that's fine, but they
> don't get to be maintainers and claim `Supported`.

What I'm actually trying to articulate is a way of measuring value of
the patch vs cost ... it has nothing really to do with who foots the
actual bill.

One thesis I'm actually starting to formulate is that this continual
devaluing of maintainers is why we have so much difficulty keeping and
recruiting them.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Rafael J. Wysocki]

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On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Mon, 2020-11-23 at 15:19 +0100, Miguel Ojeda wrote:
> > On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:

[cut]

> >
> > Maintainers routinely review 1-line trivial patches, not to mention
> > internal API changes, etc.
>
> We're also complaining about the inability to recruit maintainers:
>
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
>
> And burn out:
>
> http://antirez.com/news/129

Right.

> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches ... I'm pushing back
> on that assumption in two places, firstly the valulessness of the time
> and secondly that all trivial patches are valuable.
>
> > If some company does not want to pay for that, that's fine, but they
> > don't get to be maintainers and claim `Supported`.
>
> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.
>
> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

Absolutely.

This is just one of the factors involved, but a significant one IMV.

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

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On Mon, 2020-11-23 at 07:58 -0800, James Bottomley wrote:
> We're also complaining about the inability to recruit maintainers:
> 
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
> 
> And burn out:
> 
> http://antirez.com/news/129

https://www.wired.com/story/open-source-coders-few-tired/

> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.

It's unclear how to measure value in consistency.

But one way that costs can be reduced is by automation and _not_
involving maintainers when the patch itself is provably correct.

> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

The linux kernel has something like 1500 different maintainers listed
in the MAINTAINERS file.  That's not a trivial number.

$ git grep '^M:' MAINTAINERS | sort | uniq -c | wc -l
1543
$ git grep '^M:' MAINTAINERS| cut -f1 -d'<' | sort | uniq -c | wc -l
1446

I think the question you are asking is about trust and how it
effects development.

And back to that wired story, the actual number of what you might
be considering to be maintainers is likely less than 10% of the
listed numbers above.

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, I used git.  It says that as of today in Linus' tree we have 889
> patches related to fall throughs and the first series went in in
> october 2017 ... ignoring a couple of outliers back to February.

I can see ~10k insertions over ~1k commits and 15 years that mention a
fallthrough in the entire repo. That is including some commits (like
the biggest one, 960 insertions) that have nothing to do with C
fallthrough. A single kernel release has an order of magnitude more
changes than this...

But if we do the math, for an author, at even 1 minute per line change
and assuming nothing can be automated at all, it would take 1 month of
work. For maintainers, a couple of trivial lines is noise compared to
many other patches.

In fact, this discussion probably took more time than the time it
would take to review the 200 lines. :-)

> We're also complaining about the inability to recruit maintainers:
>
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
>
> And burn out:
>
> http://antirez.com/news/129

Accepting trivial and useful 1-line patches is not what makes a
voluntary maintainer quit... Thankless work with demanding deadlines is.

> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches

I have not said that, at all. In fact, I am a voluntary one and I
welcome patches like this. It takes very little effort on my side to
review and it helps the kernel overall. Paid maintainers are the ones
that can take care of big features/reviews.

> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.

I understand your point, but you were the one putting it in terms of a
junior FTE. In my view, 1 month-work (worst case) is very much worth
removing a class of errors from a critical codebase.

> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

That may very well be true, but I don't feel anybody has devalued
maintainers in this discussion.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Jason Gunthorpe]

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On Fri, Nov 20, 2020 at 12:21:39PM -0600, Gustavo A. R. Silva wrote:

>   IB/hfi1: Fix fall-through warnings for Clang
>   IB/mlx4: Fix fall-through warnings for Clang
>   IB/qedr: Fix fall-through warnings for Clang
>   RDMA/mlx5: Fix fall-through warnings for Clang

I picked these four to the rdma tree, thanks

Jason

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[James Bottomley]

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On Mon, 2020-11-23 at 19:56 +0100, Miguel Ojeda wrote:
> On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > Well, I used git.  It says that as of today in Linus' tree we have
> > 889 patches related to fall throughs and the first series went in
> > in october 2017 ... ignoring a couple of outliers back to February.
> 
> I can see ~10k insertions over ~1k commits and 15 years that mention
> a fallthrough in the entire repo. That is including some commits
> (like the biggest one, 960 insertions) that have nothing to do with C
> fallthrough. A single kernel release has an order of magnitude more
> changes than this...
> 
> But if we do the math, for an author, at even 1 minute per line
> change and assuming nothing can be automated at all, it would take 1
> month of work. For maintainers, a couple of trivial lines is noise
> compared to many other patches.

So you think a one line patch should take one minute to produce ... I
really don't think that's grounded in reality.  I suppose a one line
patch only takes a minute to merge with b4 if no-one reviews or tests
it, but that's not really desirable.

> In fact, this discussion probably took more time than the time it
> would take to review the 200 lines. :-)

I'm framing the discussion in terms of the whole series of changes we
have done for fall through, both what's in the tree currently (889
patches) both in terms of the produce and the consumer.  That's what I
used for my figures for cost.

> > We're also complaining about the inability to recruit maintainers:
> > 
> > https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
> > 
> > And burn out:
> > 
> > http://antirez.com/news/129
> 
> Accepting trivial and useful 1-line patches

Part of what I'm trying to measure is the "and useful" bit because
that's not a given.

> is not what makes a voluntary maintainer quit...

so the proverb "straw which broke the camel's back" uniquely doesn't
apply to maintainers

>  Thankless work with demanding deadlines is.

That's another potential reason, but it doesn't may other reasons less
valid.

> > The whole crux of your argument seems to be maintainers' time isn't
> > important so we should accept all trivial patches
> 
> I have not said that, at all. In fact, I am a voluntary one and I
> welcome patches like this. It takes very little effort on my side to
> review and it helps the kernel overall.

Well, you know, subsystems are very different in terms of the amount of
patches a maintainer has to process per release cycle of the kernel. 
If a maintainer is close to capacity, additional patches, however
trivial, become a problem.  If a maintainer has spare cycles, trivial
patches may look easy.

> Paid maintainers are the ones that can take care of big
> features/reviews.
> 
> > What I'm actually trying to articulate is a way of measuring value
> > of the patch vs cost ... it has nothing really to do with who foots
> > the actual bill.
> 
> I understand your point, but you were the one putting it in terms of
> a junior FTE.

No, I evaluated the producer side in terms of an FTE.  What we're
mostly arguing about here is the consumer side: the maintainers and
people who have to rework their patch sets. I estimated that at 100h.

>  In my view, 1 month-work (worst case) is very much worth
> removing a class of errors from a critical codebase.
> 
> > One thesis I'm actually starting to formulate is that this
> > continual devaluing of maintainers is why we have so much
> > difficulty keeping and recruiting them.
> 
> That may very well be true, but I don't feel anybody has devalued
> maintainers in this discussion.

You seem to be saying that because you find it easy to merge trivial
patches, everyone should.  I'm reminded of a friend long ago who
thought being a Tees River Pilot was a sinecure because he could
navigate the Tees blindfold.  What he forgot, of course, is that just
because it's easy with a trawler doesn't mean it's easy with an oil
tanker.  In fact it takes longer to qualify as a Tees River Pilot than
it does to get a PhD.

James

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Mark Brown]

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On Fri, 20 Nov 2020 12:21:39 -0600, Gustavo A. R. Silva wrote:
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator.git for-next

Thanks!

[1/1] regulator: as3722: Fix fall-through warnings for Clang
      commit: b52b417ccac4fae5b1f2ec4f1d46eb91e4493dc5

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Finn Thain]

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On Mon, 23 Nov 2020, Miguel Ojeda wrote:

> On Mon, 23 Nov 2020, Finn Thain wrote:
> 
> > On Sun, 22 Nov 2020, Miguel Ojeda wrote:
> > 
> > > 
> > > It isn't that much effort, isn't it? Plus we need to take into 
> > > account the future mistakes that it might prevent, too.
> > 
> > We should also take into account optimisim about future improvements 
> > in tooling.
> > 
> Not sure what you mean here. There is no reliable way to guess what the 
> intention was with a missing fallthrough, even if you parsed whitespace 
> and indentation.
> 

What I meant was that you've used pessimism as if it was fact.

For example, "There is no way to guess what the effect would be if the 
compiler trained programmers to add a knee-jerk 'break' statement to avoid 
a warning".

Moreover, what I meant was that preventing programmer mistakes is a 
problem to be solved by development tools. The idea that retro-fitting new 
language constructs onto mature code is somehow necessary to "prevent 
future mistakes" is entirely questionable.

> > > So even if there were zero problems found so far, it is still a 
> > > positive change.
> > > 
> > 
> > It is if you want to spin it that way.
> > 
> How is that a "spin"? It is a fact that we won't get *implicit* 
> fallthrough mistakes anymore (in particular if we make it a hard error).
> 

Perhaps "handwaving" is a better term?

> > > I would agree if these changes were high risk, though; but they are 
> > > almost trivial.
> > > 
> > 
> > This is trivial:
> > 
> >  case 1:
> >         this();
> > +       fallthrough;
> >  case 2:
> >         that();
> > 
> > But what we inevitably get is changes like this:
> > 
> >  case 3:
> >         this();
> > +       break;
> >  case 4:
> >         hmmm();
> > 
> > Why? Mainly to silence the compiler. Also because the patch author 
> > argued successfully that they had found a theoretical bug, often in 
> > mature code.
> > 
> If someone changes control flow, that is on them. Every kernel developer 
> knows what `break` does.
> 

Sure. And if you put -Wimplicit-fallthrough into the Makefile and if that 
leads to well-intentioned patches that cause regressions, it is partly on 
you.

Have you ever considered the overall cost of the countless 
-Wpresume-incompetence flags?

Perhaps you pay the power bill for a build farm that produces logs that 
no-one reads? Perhaps you've run git bisect, knowing that the compiler 
messages are not interesting? Or compiled software in using a language 
that generates impenetrable messages? If so, here's a tip:

# grep CFLAGS /etc/portage/make.conf 
CFLAGS="... -Wno-all -Wno-extra ..."
CXXFLAGS="${CFLAGS}"

Now allow me some pessimism: the hardware upgrades, gigawatt hours and 
wait time attributable to obligatory static analyses are a net loss.

> > But is anyone keeping score of the regressions? If unreported bugs 
> > count, what about unreported regressions?
> > 
> Introducing `fallthrough` does not change semantics. If you are really 
> keen, you can always compare the objects because the generated code 
> shouldn't change.
> 

No, it's not for me to prove that such patches don't affect code 
generation. That's for the patch author and (unfortunately) for reviewers.

> Cheers,
> Miguel
> 

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

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On Tue, 2020-11-24 at 11:58 +1100, Finn Thain wrote:
> it's not for me to prove that such patches don't affect code 
> generation. That's for the patch author and (unfortunately) for reviewers.

Ideally, that proof would be provided by the compilation system itself
and not patch authors nor reviewers nor maintainers.

Unfortunately gcc does not guarantee repeatability or deterministic output.
To my knowledge, neither does clang.

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Nick Desaulniers]

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On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > If none of the 140 patches here fix a real bug, and there is no change
> > to machine code then it sounds to me like a W=2 kind of a warning.
>
> FWIW, this series has found at least one bug so far:
> https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/

So looks like the bulk of these are:
switch (x) {
  case 0:
    ++x;
  default:
    break;
}

I have a patch that fixes those up for clang:
https://reviews.llvm.org/D91895

There's 3 other cases that don't quite match between GCC and Clang I
observe in the kernel:
switch (x) {
  case 0:
    ++x;
  default:
    goto y;
}
y:;

switch (x) {
  case 0:
    ++x;
  default:
    return;
}

switch (x) {
  case 0:
    ++x;
  default:
    ;
}

Based on your link, and Nathan's comment on my patch, maybe Clang
should continue to warn for the above (at least the `default: return;`
case) and GCC should change?  While the last case looks harmless,
there were only 1 or 2 across the tree in my limited configuration
testing; I really think we should just add `break`s for those.
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Jakub Kicinski]

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On Mon, 23 Nov 2020 17:32:51 -0800 Nick Desaulniers wrote:
> On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
> > On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:  
> > > If none of the 140 patches here fix a real bug, and there is no change
> > > to machine code then it sounds to me like a W=2 kind of a warning.  
> >
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/  
> 
> So looks like the bulk of these are:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     break;
> }
> 
> I have a patch that fixes those up for clang:
> https://reviews.llvm.org/D91895

😍

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Finn Thain]

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On Mon, 23 Nov 2020, Joe Perches wrote:

> On Tue, 2020-11-24 at 11:58 +1100, Finn Thain wrote:
> > it's not for me to prove that such patches don't affect code 
> > generation. That's for the patch author and (unfortunately) for 
> > reviewers.
> 
> Ideally, that proof would be provided by the compilation system itself 
> and not patch authors nor reviewers nor maintainers.
> 
> Unfortunately gcc does not guarantee repeatability or deterministic 
> output. To my knowledge, neither does clang.
> 

Yes, I've said the same thing myself. But having attempted it, I now think 
this is a hard problem. YMMV.

https://lore.kernel.org/linux-scsi/alpine.LNX.2.22.394.2004281017310.12(a)nippy.intranet/
https://lore.kernel.org/linux-scsi/alpine.LNX.2.22.394.2005211358460.8(a)nippy.intranet/

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

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On Mon, Nov 23, 2020 at 04:03:45PM -0400, Jason Gunthorpe wrote:
> On Fri, Nov 20, 2020 at 12:21:39PM -0600, Gustavo A. R. Silva wrote:
> 
> >   IB/hfi1: Fix fall-through warnings for Clang
> >   IB/mlx4: Fix fall-through warnings for Clang
> >   IB/qedr: Fix fall-through warnings for Clang
> >   RDMA/mlx5: Fix fall-through warnings for Clang
> 
> I picked these four to the rdma tree, thanks

Awesome. :)

Thank you, Jason.
--
Gustavo

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

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On Mon, Nov 23, 2020 at 08:38:46PM +0000, Mark Brown wrote:
> On Fri, 20 Nov 2020 12:21:39 -0600, Gustavo A. R. Silva wrote:
> > This series aims to fix almost all remaining fall-through warnings in
> > order to enable -Wimplicit-fallthrough for Clang.
> > 
> > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > add multiple break/goto/return/fallthrough statements instead of just
> > letting the code fall through to the next case.
> > 
> > [...]
> 
> Applied to
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator.git for-next
> 
> Thanks!
> 
> [1/1] regulator: as3722: Fix fall-through warnings for Clang
>       commit: b52b417ccac4fae5b1f2ec4f1d46eb91e4493dc5

Thank you, Mark.
--
Gustavo

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

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On Mon, Nov 23, 2020 at 05:32:51PM -0800, Nick Desaulniers wrote:
> On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > > If none of the 140 patches here fix a real bug, and there is no change
> > > to machine code then it sounds to me like a W=2 kind of a warning.
> >
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/
> 
> So looks like the bulk of these are:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     break;
> }
> 
> I have a patch that fixes those up for clang:
> https://reviews.llvm.org/D91895

I still think this isn't right -- it's a case statement that runs off
the end without an explicit flow control determination. I think Clang is
right to warn for these, and GCC should also warn.

-- 
Kees Cook

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Tue, Nov 24, 2020 at 1:58 AM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> What I meant was that you've used pessimism as if it was fact.

"future mistakes that it might prevent" is neither pessimism nor states a fact.

> For example, "There is no way to guess what the effect would be if the
> compiler trained programmers to add a knee-jerk 'break' statement to avoid
> a warning".

It is only knee-jerk if you think you are infallible.

> Moreover, what I meant was that preventing programmer mistakes is a
> problem to be solved by development tools

This warning comes from a development tool -- the compiler.

> The idea that retro-fitting new
> language constructs onto mature code is somehow necessary to "prevent
> future mistakes" is entirely questionable.

The kernel is not a frozen codebase.

Further, "mature code vs. risk of change" arguments don't apply here
because the semantics of the program and binary output isn't changing.

> Sure. And if you put -Wimplicit-fallthrough into the Makefile and if that
> leads to well-intentioned patches that cause regressions, it is partly on
> you.

Again: adding a `fallthrough` does not change the program semantics.
If you are a maintainer and want to cross-check, compare the codegen.

> Have you ever considered the overall cost of the countless
> -Wpresume-incompetence flags?

Yeah: negative. On the other hand, the overall cost of the countless
-fI-am-infallible flags is very noticeable.

> Perhaps you pay the power bill for a build farm that produces logs that
> no-one reads? Perhaps you've run git bisect, knowing that the compiler
> messages are not interesting? Or compiled software in using a language
> that generates impenetrable messages? If so, here's a tip:
>
> # grep CFLAGS /etc/portage/make.conf
> CFLAGS="... -Wno-all -Wno-extra ..."
> CXXFLAGS="${CFLAGS}"
>
> Now allow me some pessimism: the hardware upgrades, gigawatt hours and
> wait time attributable to obligatory static analyses are a net loss.

If you really believe compiler warnings and static analysis are
useless and costly, I think there is not much point in continuing the
discussion.

> No, it's not for me to prove that such patches don't affect code
> generation. That's for the patch author and (unfortunately) for reviewers.

I was not asking you to prove it. I am stating that proving it is very easy.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Mon, Nov 23, 2020 at 9:38 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> So you think a one line patch should take one minute to produce ... I
> really don't think that's grounded in reality.

No, I have not said that. Please don't put words in my mouth (again).

I have said *authoring* lines of *this* kind takes a minute per line.
Specifically: lines fixing the fallthrough warning mechanically and
repeatedly where the compiler tells you to, and doing so full-time for
a month.

For instance, take the following one from Gustavo. Are you really
saying it takes 12 minutes (your number) to write that `break;`?

diff --git a/drivers/gpu/drm/via/via_irq.c b/drivers/gpu/drm/via/via_irq.c
index 24cc445169e2..a3e0fb5b8671 100644
--- a/drivers/gpu/drm/via/via_irq.c
+++ b/drivers/gpu/drm/via/via_irq.c
@@ -364,6 +364,7 @@ int via_wait_irq(struct drm_device *dev, void
*data, struct drm_file *file_priv)
                irqwait->request.sequence +=
                        atomic_read(&cur_irq->irq_received);
                irqwait->request.type &= ~_DRM_VBLANK_RELATIVE;
+               break;
        case VIA_IRQ_ABSOLUTE:
                break;
        default:

>  I suppose a one line
> patch only takes a minute to merge with b4 if no-one reviews or tests
> it, but that's not really desirable.

I have not said that either. I said reviewing and merging those are
noise compared to any complex patch. Testing should be done by the
author comparing codegen.

> Part of what I'm trying to measure is the "and useful" bit because
> that's not a given.

It is useful since it makes intent clear. It also catches actual bugs,
which is even more valuable.

> Well, you know, subsystems are very different in terms of the amount of
> patches a maintainer has to process per release cycle of the kernel.
> If a maintainer is close to capacity, additional patches, however
> trivial, become a problem.  If a maintainer has spare cycles, trivial
> patches may look easy.

First of all, voluntary maintainers choose their own workload.
Furthermore, we already measure capacity in the `MAINTAINERS` file:
maintainers can state they can only handle a few patches. Finally, if
someone does not have time for a trivial patch, they are very unlikely
to have any time to review big ones.

> You seem to be saying that because you find it easy to merge trivial
> patches, everyone should.

Again, I have not said anything of the sort.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Sean Young]

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On Mon, Nov 23, 2020 at 07:58:06AM -0800, James Bottomley wrote:
> On Mon, 2020-11-23 at 15:19 +0100, Miguel Ojeda wrote:
> > On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> > > It's not about the risk of the changes it's about the cost of
> > > implementing them.  Even if you discount the producer time (which
> > > someone gets to pay for, and if I were the engineering manager, I'd
> > > be unhappy about), the review/merge/rework time is pretty
> > > significant in exchange for six minor bug fixes.  Fine, when a new
> > > compiler warning comes along it's certainly reasonable to see if we
> > > can benefit from it and the fact that the compiler people think
> > > it's worthwhile is enough evidence to assume this initially.  But
> > > at some point you have to ask whether that assumption is supported
> > > by the evidence we've accumulated over the time we've been using
> > > it.  And if the evidence doesn't support it perhaps it is time to
> > > stop the experiment.
> > 
> > Maintainers routinely review 1-line trivial patches, not to mention
> > internal API changes, etc.
> 
> We're also complaining about the inability to recruit maintainers:
> 
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
> 
> And burn out:
> 
> http://antirez.com/news/129
> 
> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches ... I'm pushing back
> on that assumption in two places, firstly the valulessness of the time
> and secondly that all trivial patches are valuable.

You're assuming burn out or recruitment problems is due to patch workload
or too many "trivial" patches.

In my experience, "other maintainers" is by far the biggest cause of
burn out for my kernel maintenance work.

Certainly arguing with a maintainer about some obviously-correct patch
series must be a good example of this.


Sean

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Andy Shevchenko]

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On Mon, Nov 23, 2020 at 10:39 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
> On Mon, 2020-11-23 at 19:56 +0100, Miguel Ojeda wrote:
> > On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:

...

> > But if we do the math, for an author, at even 1 minute per line
> > change and assuming nothing can be automated at all, it would take 1
> > month of work. For maintainers, a couple of trivial lines is noise
> > compared to many other patches.
>
> So you think a one line patch should take one minute to produce ... I
> really don't think that's grounded in reality.  I suppose a one line
> patch only takes a minute to merge with b4 if no-one reviews or tests
> it, but that's not really desirable.

In my practice most of the one line patches were either to fix or to
introduce quite interesting issues.
1 minute is 2-3 orders less than usually needed for such patches.
That's why I don't like churn produced by people who often even didn't
compile their useful contributions.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Edward Cree]

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On 25/11/2020 00:32, Miguel Ojeda wrote:
> I have said *authoring* lines of *this* kind takes a minute per line.
> Specifically: lines fixing the fallthrough warning mechanically and
> repeatedly where the compiler tells you to, and doing so full-time for
> a month.
<snip>
> It is useful since it makes intent clear.
To make the intent clear, you have to first be certain that you
 understand the intent; otherwise by adding either a break or a
 fallthrough to suppress the warning you are just destroying the
 information that "the intent of this code is unknown".
Figuring out the intent of a piece of unfamiliar code takes more
 than 1 minute; just because
    case foo:
        thing;
    case bar:
        break;
 produces identical code to
    case foo:
        thing;
        break;
    case bar:
        break;
 doesn't mean that *either* is correct — maybe the author meant
 to write
    case foo:
        return thing;
    case bar:
        break;
 and by inserting that break you've destroyed the marker that
 would direct someone who knew what the code was about to look
 at that point in the code and spot the problem.
Thus, you *always* have to look at more than just the immediate
 mechanical context of the code, to make a proper judgement that
 yes, this was the intent.  If you think that that sort of thing
 can be done in an *average* time of one minute, then I hope you
 stay away from code I'm responsible for!
One minute would be an optimistic target for code that, as the
 maintainer, one is already somewhat familiar with.  For code
 that you're seeing for the first time, as is usually the case
 with the people doing these mechanical fix-a-warning patches,
 it's completely unrealistic.

A warning is only useful because it makes you *think* about the
 code.  If you suppress the warning without doing that thinking,
 then you made the warning useless; and if the warning made you
 think about code that didn't *need* it, then the warning was
 useless from the start.

So make your mind up: does Clang's stricter -Wimplicit-fallthrough
 flag up code that needs thought (in which case the fixes take
 effort both to author and to review) or does it flag up code
 that can be mindlessly "fixed" (in which case the warning is
 worthless)?  Proponents in this thread seem to be trying to
 have it both ways.

-ed

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Edward Cree]

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On 24/11/2020 21:25, Kees Cook wrote:
> I still think this isn't right -- it's a case statement that runs off
> the end without an explicit flow control determination.

Proves too much — for instance
    case foo:
    case bar:
        thing;
        break;
 doesn't require a fallthrough; after case foo:, and afaik
 no-one is suggesting it should.  Yet it, too, is "a case
 statement that runs off the end without an explicit flow
 control determination".

-ed

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@gmail.com> wrote:
>
> To make the intent clear, you have to first be certain that you
>  understand the intent; otherwise by adding either a break or a
>  fallthrough to suppress the warning you are just destroying the
>  information that "the intent of this code is unknown".

If you don't know what the intent of your own code is, then you
*already* have a problem in your hands.

> Figuring out the intent of a piece of unfamiliar code takes more
>  than 1 minute; just because
>     case foo:
>         thing;
>     case bar:
>         break;
>  produces identical code to
>     case foo:
>         thing;
>         break;
>     case bar:
>         break;
>  doesn't mean that *either* is correct — maybe the author meant

What takes 1 minute is adding it *mechanically* by the author, i.e. so
that you later compare whether codegen is the same.

>  to write
>     case foo:
>         return thing;
>     case bar:
>         break;
>  and by inserting that break you've destroyed the marker that
>  would direct someone who knew what the code was about to look
>  at that point in the code and spot the problem.

Then it means you already have a bug. This patchset gives the
maintainer a chance to notice it, which is a good thing. The "you've
destroyed the market" claim is bogus, because:
  1. you were not looking into it
  2. you didn't notice the bug so far
  3. is implicit -- harder to spot
  4. is only useful if you explicitly take a look at this kind of bug.
So why don't you do it now?

> Thus, you *always* have to look at more than just the immediate
>  mechanical context of the code, to make a proper judgement that
>  yes, this was the intent.

I find that is the responsibility of the maintainers and reviewers for
tree-wide patches like this, assuming they want. They can also keep
the behavior (and the bugs) without spending time. Their choice.

> If you think that that sort of thing
>  can be done in an *average* time of one minute, then I hope you
>  stay away from code I'm responsible for!

Please don't accuse others of recklessness or incompetence, especially
if you didn't understand what they said.

> A warning is only useful because it makes you *think* about the
>  code.  If you suppress the warning without doing that thinking,
>  then you made the warning useless; and if the warning made you
>  think about code that didn't *need* it, then the warning was
>  useless from the start.

We are not suppressing the warning. Quite the opposite, in fact.

> So make your mind up: does Clang's stricter -Wimplicit-fallthrough
>  flag up code that needs thought (in which case the fixes take
>  effort both to author and to review)

As I said several times already, it does take time to review if the
maintainer wants to take the chance to see if they had a bug to begin
with, but it does not require thought for the author if they just go
for equivalent codegen.

> or does it flag up code
>  that can be mindlessly "fixed" (in which case the warning is
>  worthless)?  Proponents in this thread seem to be trying to
>  have it both ways.

A warning is not worthless just because you can mindlessly fix it.
There are many counterexamples, e.g. many
checkpatch/lint/lang-format/indentation warnings, functional ones like
the `if (a = b)` warning...

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Geert Uytterhoeven]

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Hi Miguel,

On Thu, Nov 26, 2020 at 3:54 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
> On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@gmail.com> wrote:
> > To make the intent clear, you have to first be certain that you
> >  understand the intent; otherwise by adding either a break or a
> >  fallthrough to suppress the warning you are just destroying the
> >  information that "the intent of this code is unknown".
>
> If you don't know what the intent of your own code is, then you
> *already* have a problem in your hands.

The maintainer is not necessarily the owner/author of the code, and
thus may not know the intent of the code.

> > or does it flag up code
> >  that can be mindlessly "fixed" (in which case the warning is
> >  worthless)?  Proponents in this thread seem to be trying to
> >  have it both ways.
>
> A warning is not worthless just because you can mindlessly fix it.
> There are many counterexamples, e.g. many
> checkpatch/lint/lang-format/indentation warnings, functional ones like
> the `if (a = b)` warning...

BTW, you cannot mindlessly fix the latter, as you cannot know if
"(a == b)" or "((a = b))" was intended, without understanding the code
(and the (possibly unavailable) data sheet, and the hardware, ...).

P.S. So far I've stayed out of this thread, as I like it if the compiler
     flags possible mistakes.  After all I was the one fixing new
     "may be used uninitialized" warnings thrown up by gcc-4.1, until
     (a bit later than) support for that compiler was removed...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert(a)linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Karol Herbst]

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On Thu, Nov 26, 2020 at 4:28 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Miguel,
>
> On Thu, Nov 26, 2020 at 3:54 PM Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
> > On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@gmail.com> wrote:
> > > To make the intent clear, you have to first be certain that you
> > >  understand the intent; otherwise by adding either a break or a
> > >  fallthrough to suppress the warning you are just destroying the
> > >  information that "the intent of this code is unknown".
> >
> > If you don't know what the intent of your own code is, then you
> > *already* have a problem in your hands.
>
> The maintainer is not necessarily the owner/author of the code, and
> thus may not know the intent of the code.
>
> > > or does it flag up code
> > >  that can be mindlessly "fixed" (in which case the warning is
> > >  worthless)?  Proponents in this thread seem to be trying to
> > >  have it both ways.
> >
> > A warning is not worthless just because you can mindlessly fix it.
> > There are many counterexamples, e.g. many
> > checkpatch/lint/lang-format/indentation warnings, functional ones like
> > the `if (a = b)` warning...
>
> BTW, you cannot mindlessly fix the latter, as you cannot know if
> "(a == b)" or "((a = b))" was intended, without understanding the code
> (and the (possibly unavailable) data sheet, and the hardware, ...).
>

to allow assignments in if statements was clearly a mistake and if you
need outside information to understand the code, your code is the
issue already.

> P.S. So far I've stayed out of this thread, as I like it if the compiler
>      flags possible mistakes.  After all I was the one fixing new
>      "may be used uninitialized" warnings thrown up by gcc-4.1, until
>      (a bit later than) support for that compiler was removed...
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert(a)linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
> _______________________________________________
> dri-devel mailing list
> dri-devel(a)lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

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On Thu, Nov 26, 2020 at 4:28 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> The maintainer is not necessarily the owner/author of the code, and
> thus may not know the intent of the code.

Agreed, I was not blaming maintainers -- just trying to point out that
the problem is there :-)

In those cases, it is still very useful: we add the `fallthrough` and
a comment saying `FIXME: fallthrough intended? Figure this out...`.
Thus a previous unknown unknown is now a known unknown. And no new
unknown unknowns will be introduced since we enabled the warning
globally.

> BTW, you cannot mindlessly fix the latter, as you cannot know if
> "(a == b)" or "((a = b))" was intended, without understanding the code
> (and the (possibly unavailable) data sheet, and the hardware, ...).

That's right, I was referring to the cases where the compiler saves
someone time from a typo they just made.

Cheers,
Miguel

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Martin K. Petersen]

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Gustavo,

> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.

Applied 20-22,54,120-124 to 5.11/scsi-staging, thanks.

-- 
Martin K. Petersen	Oracle Linux Engineering

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

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On Tue, Dec 01, 2020 at 12:52:27AM -0500, Martin K. Petersen wrote:
> 
> Gustavo,
> 
> > This series aims to fix almost all remaining fall-through warnings in
> > order to enable -Wimplicit-fallthrough for Clang.
> 
> Applied 20-22,54,120-124 to 5.11/scsi-staging, thanks.

Awesome! :)

Thanks, Martin.
--
Gustavo

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Dan Carpenter]

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On Sun, Nov 22, 2020 at 08:17:03AM -0800, Kees Cook wrote:
> On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > > This series aims to fix almost all remaining fall-through warnings in
> > > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > > 
> > > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > > letting the code fall through to the next case.
> > > > > 
> > > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > > to move in that direction.
> > > > > 
> > > > > Something important to mention is that there is currently a discrepancy
> > > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > > statements or to cases that only contain a break/continue/return
> > > > > statement[2][3][4].  
> > > > 
> > > > Are we sure we want to make this change? Was it discussed before?
> > > > 
> > > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > > find?
> > > > 
> > > > IMVHO compiler warnings are supposed to warn about issues that could
> > > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > > 
> > > It's certainly a place where the intent is not always clear. I think
> > > this makes all the cases unambiguous, and doesn't impact the machine
> > > code, since the compiler will happily optimize away any behavioral
> > > redundancy.
> > 
> > If none of the 140 patches here fix a real bug, and there is no change
> > to machine code then it sounds to me like a W=2 kind of a warning.
> 
> FWIW, this series has found at least one bug so far:
> https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/

This is a fallthrough to a return and not to a break.  That should
trigger a warning.  The fallthrough to a break should not generate a
warning.

The bug we're trying to fix is "missing break statement" but if the
result of the bug is "we hit a break statement" then now we're just
talking about style.  GCC should limit itself to warning about
potentially buggy code.

regards,
dan carpenter

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Dan Carpenter]

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On Mon, Nov 23, 2020 at 05:32:51PM -0800, Nick Desaulniers wrote:
> On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > > If none of the 140 patches here fix a real bug, and there is no change
> > > to machine code then it sounds to me like a W=2 kind of a warning.
> >
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ(a)mail.gmail.com/
> 
> So looks like the bulk of these are:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     break;
> }

This should not generate a warning.

> 
> I have a patch that fixes those up for clang:
> https://reviews.llvm.org/D91895
> 
> There's 3 other cases that don't quite match between GCC and Clang I
> observe in the kernel:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     goto y;
> }
> y:;

This should generate a warning.

> 
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     return;
> }

Warn for this.


> 
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     ;
> }

Don't warn for this.

If adding a break statement changes the flow of the code then warn about
potentially missing break statements, but if it doesn't change anything
then don't warn about it.

regards,
dan carpenter