[PATCH printk v2 00/38] reduce console_lock scope

John Ogness
2022-10-19T15:03:12Z

This is v2 of a series to prepare for threaded/atomic
printing. It is a rework of patches 6-12 of the v1 [0]. From
the v1, patches 1-5 are already mainline and a rework of
patches >12 will be posted in a later series.

This series focuses on reducing the scope of the BKL
console_lock. It achieves this by switching to SRCU and a
dedicated mutex for console list iteration and modification,
respectively. The console_lock will no longer offer this
protection and is completely removed from
(un)register_console() and console_stop/start() code.

All users of the console_lock for list iteration have been
modified. For the call sites where the console_lock is still
needed (because of other reasons), I added comments to explain
exactly why the console_lock was needed.

The base commit for this series is from Paul McKenney's RCU tree
and provides an NMI-safe SRCU implementation [1]. Without the
NMI-safe SRCU implementation, this series is not less safe than
mainline. But we will need the NMI-safe SRCU implementation for
atomic consoles anyway, so we might as well get it in
now. Especially since it _does_ increase the reliability for
mainline in the panic path.

Changes since v2:

general:

- introduce console_is_enabled() to document safe data race on
console->flags

- switch all "console->flags & CON_ENABLED" code sites to
console_is_enabled()

- add "for_each_console_srcu" to .clang-format

- cleanup/clarify comments relating to console_lock
coverage/usage

um:

- kmsg_dumper: use srcu instead of console_lock for list
iteration

kgdb/kdb:

- configure_kgdboc: keep console_lock for console->device()
synchronization, use srcu for list iteration

- kgdboc_earlycon_pre_exp_handler: use srcu instead of
documenting unsafety for list iteration

- kgdboc_earlycon_init: use console_list_lock instead of
console_lock to lock list

- kdb_msg_write: use srcu instead of documenting unsafety for
list iteration

tty:

- show_cons_active: keep console_lock for console->device()
synchronization

fbdev:

- xen-fbfront: xenfb_probe: use srcu instead of console_lock
for list iteration, introduce console_force_preferred() to
safely implement hack

proc/consoles:

- show_console_dev: keep console_lock for console->device()
synchronization

- c_next: use hlist_entry_safe() instead of
hlist_for_each_entry_continue()

printk:

- remove console_lock from console_stop/start() and
(un)register_console()

- introduce console_srcu_read_(un)lock() to wrap scru read
(un)lock

- rename cons_first() macro to console_first()

- for_each_console: add lockdep check instead of introducing
new for_each_registered_console()

- console_list_lock: add warning if in read-side critical
section

- release srcu read lock on handover

- console_flush_all: use srcu instead of relying on console
lock for list iteration

- console_unblank: use srcu instead of relying on console_lock
for list iteration

- console_flush_on_panic: use srcu for list iteration and
document console->seq race

- device: keep console_lock for console->device()
synchronization, usr srcu for list iteration

- register_console: split list adding logic into the 3 distinct
scenarios

- register_console: set initial sequence number before adding
to list

- unregister_console: fix ENODEV return value if the console is
not registered

- console_stop: synchronize srcu

- printk_late_init: use _safe variant of iteration

- __pr_flush: use srcu instead of relying on console_lock for
list iteration

John Ogness

[0] https://lore.kernel.org/r/20220924000454.3319186-1-john.ogness@linutronix.de
[1] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/log/?h=srcunmisafe.2022.10.18b

John Ogness (37):
printk: Convert console_drivers list to hlist
printk: Prepare for SRCU console list protection
printk: introduce console_is_enabled() wrapper
printk: use console_is_enabled()
tty: nfcon: use console_is_enabled()
um: kmsg_dump: use console_is_enabled()
efi: earlycon: use console_is_enabled()
netconsole: use console_is_enabled()
tty: hvc: use console_is_enabled()
tty: serial: earlycon: use console_is_enabled()
tty: serial: kgdboc: use console_is_enabled()
tty: serial: pic32_uart: use console_is_enabled()
tty: serial: samsung_tty: use console_is_enabled()
tty: serial: serial_core: use console_is_enabled()
tty: serial: xilinx_uartps: use console_is_enabled()
tty: tty_io: use console_is_enabled()
usb: early: xhci-dbc: use console_is_enabled()
kdb: kdb_io: use console_is_enabled()
um: kmsg_dumper: use srcu console list iterator
serial: kgdboc: use srcu console list iterator
serial: kgdboc: document console_lock usage
tty: tty_io: document console_lock usage
xen: fbfront: use srcu console list iterator
proc: consoles: document console_lock usage
kdb: use srcu console list iterator
printk: console_flush_all: use srcu console list iterator
printk: console_unblank: use srcu console list iterator
printk: console_flush_on_panic: use srcu console list iterator
printk: console_device: use srcu console list iterator
printk: register_console: use srcu console list iterator
printk: __pr_flush: use srcu console list iterator
printk: introduce console_list_lock
serial: kgdboc: use console_list_lock instead of console_lock
tty: tty_io: use console_list_lock for list synchronization
proc: consoles: use console_list_lock for list iteration
printk: relieve console_lock of list synchronization duties
printk, xen: fbfront: create/use safe function for forcing preferred

Thomas Gleixner (1):
serial: kgdboc: Lock console list in probe function

.clang-format | 1 +
arch/m68k/emu/nfcon.c | 4 +-
arch/um/kernel/kmsg_dump.c | 15 +-
drivers/firmware/efi/earlycon.c | 4 +-
drivers/net/netconsole.c | 4 +-
drivers/tty/hvc/hvc_console.c | 2 +-
drivers/tty/serial/earlycon.c | 4 +-
drivers/tty/serial/kgdboc.c | 37 ++-
drivers/tty/serial/pic32_uart.c | 2 +-
drivers/tty/serial/samsung_tty.c | 2 +-
drivers/tty/serial/serial_core.c | 2 +-
drivers/tty/serial/xilinx_uartps.c | 2 +-
drivers/tty/tty_io.c | 18 +-
drivers/usb/early/xhci-dbc.c | 2 +-
drivers/video/fbdev/xen-fbfront.c | 16 +-
fs/proc/consoles.c | 20 +-
include/linux/console.h | 75 +++++-
include/linux/serial_core.h | 2 +-
kernel/debug/kdb/kdb_io.c | 7 +-
kernel/printk/printk.c | 373 +++++++++++++++++++++--------
20 files changed, 438 insertions(+), 154 deletions(-)

base-commit: c2d158a284abd63d727dad7402a2eed650dd4233
--
2.30.2