Re: [PATCH v7 04/10] selftests/harness: Fix interleaved scheduling leading to race conditions

[Mark Brown <broonie@kernel.org>]

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On Sat, May 11, 2024 at 07:14:39PM +0200, Mickaël Salaün wrote:

> Fix a race condition when running several FIXTURE_TEARDOWN() managing
> the same resource.  This fixes a race condition in the Landlock file
> system tests when creating or unmounting the same directory.

> Using clone3() with CLONE_VFORK guarantees that the child and grandchild
> test processes are sequentially scheduled.  This is implemented with a
> new clone3_vfork() helper replacing the fork() call.

This is now in mainline and appears to be causing several tests (at
least the ptrace vmaccess global_attach test on arm64, possibly also
some of the epoll tests) that previously were timed out by the harness
to to hang instead.  A bisect seems to point at this patch in
particular, there was a bunch of discussion of the fallout of these
patches but I'm afraid I lost track of it, is there something in flight
for this?  -next is affected as well from the looks of it.

Log of the ptrace issue (not that it's useful, the job just gets killed
by the test runner):

   https://lava.sirena.org.uk/scheduler/job/307984

Bisect log:

git bisect start
# status: waiting for both good and bad commits
# good: [e8f897f4afef0031fe618a8e94127a0934896aba] Linux 6.8
git bisect good e8f897f4afef0031fe618a8e94127a0934896aba
# status: waiting for bad commit, 1 good commit known
# bad: [a38297e3fb012ddfa7ce0321a7e5a8daeb1872b6] Linux 6.9
git bisect bad a38297e3fb012ddfa7ce0321a7e5a8daeb1872b6
# good: [480e035fc4c714fb5536e64ab9db04fedc89e910] Merge tag 'drm-next-2024-03-13' of https://gitlab.freedesktop.org/drm/kernel
git bisect good 480e035fc4c714fb5536e64ab9db04fedc89e910
# good: [2ac2b1665d3fbec6ca709dd6ef3ea05f4a51ee4c] Merge tag 'hwlock-v6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/remoteproc/linux
git bisect good 2ac2b1665d3fbec6ca709dd6ef3ea05f4a51ee4c
# good: [e858beeddfa3a400844c0e22d2118b3b52f1ea5e] bcachefs: If we run merges at a lower watermark, they must be nonblocking
git bisect good e858beeddfa3a400844c0e22d2118b3b52f1ea5e
# good: [e43afae4a335ac0bf54c7a8f23ed65dd55449649] Merge tag 'powerpc-6.9-3' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux
git bisect good e43afae4a335ac0bf54c7a8f23ed65dd55449649
# good: [09e10499ee6a5a89fc352f25881276398a49596a] Merge tag 'drm-misc-fixes-2024-05-02' of https://gitlab.freedesktop.org/drm/misc/kernel into drm-fixes
git bisect good 09e10499ee6a5a89fc352f25881276398a49596a
# good: [3c15237018eb8a9a56bb49a5dbf4d8eeb154b5cc] Merge tag 'usb-6.9-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb
git bisect good 3c15237018eb8a9a56bb49a5dbf4d8eeb154b5cc
# good: [62788b0f225da1837ad38101112e2c49123470ee] Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rmk/linux
git bisect good 62788b0f225da1837ad38101112e2c49123470ee
# good: [ed44935c330a2633440e8d2660db3c7538eeaf10] Merge tag 'spi-fix-v6.9-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi
git bisect good ed44935c330a2633440e8d2660db3c7538eeaf10
# good: [c22c3e0753807feee1391a22228b0d5e6ba39b74] Merge tag 'mm-hotfixes-stable-2024-05-10-13-14' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
git bisect good c22c3e0753807feee1391a22228b0d5e6ba39b74
# good: [b61821bb32c5577272408e1b05e6a0879a64257f] Merge tag 'drm-misc-fixes-2024-05-10' of https://gitlab.freedesktop.org/drm/misc/kernel into drm-fixes
git bisect good b61821bb32c5577272408e1b05e6a0879a64257f
# bad: [323feb3bdb67649bfa5614eb24ec9cb92a60cf33] selftests/harness: Handle TEST_F()'s explicit exit codes
git bisect bad 323feb3bdb67649bfa5614eb24ec9cb92a60cf33
# bad: [323feb3bdb67649bfa5614eb24ec9cb92a60cf33] selftests/harness: Handle TEST_F()'s explicit exit codes
git bisect bad 323feb3bdb67649bfa5614eb24ec9cb92a60cf33
# bad: [3656bc23429a4d539c81b5cb8f17ceeeeca8901a] selftests/landlock: Do not allocate memory in fixture data
git bisect bad 3656bc23429a4d539c81b5cb8f17ceeeeca8901a
# good: [7e4042abe2ee7c0977fd8bb049a6991b174a5e6f] selftests/landlock: Fix FS tests when run on a private mount point
git bisect good 7e4042abe2ee7c0977fd8bb049a6991b174a5e6f
# bad: [a86f18903db9211e265cc130b61adb175b7a4c42] selftests/harness: Fix interleaved scheduling leading to race conditions
git bisect bad a86f18903db9211e265cc130b61adb175b7a4c42
# good: [fff37bd32c7605d93bf900c4c318d56d12000048] selftests/harness: Fix fixture teardown
git bisect good fff37bd32c7605d93bf900c4c318d56d12000048
# first bad commit: [a86f18903db9211e265cc130b61adb175b7a4c42] selftests/harness: Fix interleaved scheduling leading to race conditions

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