[PATCH v8 00/12] syfs: generic deadlock fix with module removal

Luis Chamberlain
2021-09-27T16:38:31Z

This is a follow up to my v7 series of fixes for the zram driver [0]
which ended up uncovering a generic deadlock issue with sysfs and module
removal. I've reported this issue and proposed a few patches first since
March 2021 [1]. At the end of this email you will find an itemized list
of changes since that v1 series, you can also find these changes on my
branch 20210927-sysfs-generic-deadlock-fix [4] which is based on
linux-next tag next-20210927.

Just a heads up, I'm goin on vacation in two days, won't be back until
Monday October 11th.

On this v8 I incorporate feedback from the v7 series, namely:

- Tejun requested I move the struct module to the last attribute when
extending functions
- As per discussion with Tejun, trimmed and clarified the commit log
and documentation on the generic fix on patch 7
- As requested by Bart Van Assche, I simplied the setting of the
struct test_config *config into one line instead of two on many
places on patch 3 which adds the new sysfs selftest
- Dan Williams had some questions about patch 7, and so clarified these
questions using a more elaborate example on the commit log to show
where the lock call was happening.
- Trimmed the Cc list considerably as it was way too long before
- Rebased onto linux-next tag next-20210927

Below a list of changes of this patch set since its inception:

On v1:
- Open coded the sysfs deadlock race to only be localized by the zram
driver
Changes on v2:
- used bdgrab() as well for another race which was speculated by
Minchan
- improved documentation of fixes
Changes on v3:
- used a localized zram macros for the sysfs attributes instead of
open coding on each routine
- replaced bdget() stuff for a generic get_device() and bus_get() on
dev_attr_show() / dev_attr_store() for the issue speculated by
Michan
Changes on v4:
- Cosmetic fixes on the zram fixes as requested by Greg
- Split out the driver core fix as requested by Greg for the
issue speculated by Michan. This fix ended up getting up to its 4th
patch iteration [2] and eventually hit linux-next. We got a 0day
0day suspend stres fail for this patch [3]
Changes on v5:
- I ended up writing a test_sysfs driver and with it I ended up
proving that the issue speculated by Michen was not possible and
so I asked Greg to drop the patch from his queue titled
"sysfs: fix kobject refcount to address races with kobject removal"
- checkpatch fixes for the zram changes
Changes on v6:
- I submitted my test_sysfs driver for inclusion upstream which easily
abstracted the deadlock issue in a driver generically [4]
- I rebased the zram fixes and added also a new patch for zram to use
ATTRIBUTE_GROUPS As per Minchen I sent the patches to be merged
through Andrew Morton.
- Greg ended up NACK'ing the patchset because he was not sure the fix
was correct still
Changes on v7:
- Formalizes the original proposed generic sysfs fix intead of using
macro helpers to work around the issue
- I decided it is best to merge all the effort together into
one patch set because communication was being lost when I split the
patches up. This was not helping in any way to either fix the zram
issues or come to consensus on a generic solution. The patches are
also merged now because they are all related now.
- Running checkpatch exposed that S_IRWXUGO and S_IRWXU|S_IRUGO|S_IXUGO
should be replaced, so I did that in this series in two new patches
- Adds a try_module_get() documentation extension with tribal
knowledge and new information I don't think some folks still believe
in. The new test_sysfs selftest however proves this information to
be correct, the same selftest can be used to try to prove that
documentation incorrect
- Because the fix is now generic zram's deadlock can easily be fixed
now by just making it use ATTRIBUTE_GROUPS().

[0] https://lkml.kernel.org/r/YUjLAbnEB5qPfnL8@slm.duckdns.org
[1] https://lkml.kernel.org/r/20210306022035.11266-1-mcgrof@kernel.org
[2] https://lkml.kernel.org/r/20210623215007.862787-1-mcgrof@kernel.org
[3] https://lkml.kernel.org/r/20210701022737.GC21279@xsang-OptiPlex-9020
[4] https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux-next.git/log/?h=20210927-sysfs-generic-deadlock-fix

Luis Chamberlain (12):
LICENSES: Add the copyleft-next-0.3.1 license
testing: use the copyleft-next-0.3.1 SPDX tag
selftests: add tests_sysfs module
kernfs: add initial failure injection support
test_sysfs: add support to use kernfs failure injection
kernel/module: add documentation for try_module_get()
fs/kernfs/symlink.c: replace S_IRWXUGO with 0777 on
kernfs_create_link()
fs/sysfs/dir.c: replace S_IRWXU|S_IRUGO|S_IXUGO with 0755
sysfs_create_dir_ns()
sysfs: fix deadlock race with module removal
test_sysfs: enable deadlock tests by default
zram: fix crashes with cpu hotplug multistate
zram: use ATTRIBUTE_GROUPS to fix sysfs deadlock module removal

.../fault-injection/fault-injection.rst | 22 +
LICENSES/dual/copyleft-next-0.3.1 | 237 +++
MAINTAINERS | 9 +-
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 +-
drivers/block/zram/zram_drv.c | 74 +-
fs/kernfs/Makefile | 1 +
fs/kernfs/dir.c | 44 +-
fs/kernfs/failure-injection.c | 91 ++
fs/kernfs/file.c | 19 +-
fs/kernfs/kernfs-internal.h | 75 +-
fs/kernfs/symlink.c | 4 +-
fs/sysfs/dir.c | 5 +-
fs/sysfs/file.c | 6 +-
fs/sysfs/group.c | 3 +-
include/linux/kernfs.h | 19 +-
include/linux/module.h | 34 +-
include/linux/sysfs.h | 52 +-
kernel/cgroup/cgroup.c | 2 +-
lib/Kconfig.debug | 25 +
lib/Makefile | 1 +
lib/test_kmod.c | 12 +-
lib/test_sysctl.c | 12 +-
lib/test_sysfs.c | 952 ++++++++++++
tools/testing/selftests/kmod/kmod.sh | 13 +-
tools/testing/selftests/sysctl/sysctl.sh | 12 +-
tools/testing/selftests/sysfs/Makefile | 12 +
tools/testing/selftests/sysfs/config | 5 +
tools/testing/selftests/sysfs/sysfs.sh | 1383 +++++++++++++++++
28 files changed, 3026 insertions(+), 102 deletions(-)
create mode 100644 LICENSES/dual/copyleft-next-0.3.1
create mode 100644 fs/kernfs/failure-injection.c
create mode 100644 lib/test_sysfs.c
create mode 100644 tools/testing/selftests/sysfs/Makefile
create mode 100644 tools/testing/selftests/sysfs/config
create mode 100755 tools/testing/selftests/sysfs/sysfs.sh

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2.30.2