Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

[Mark Brown <broonie@kernel.org>]

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On Tue, Apr 08, 2025 at 04:58:34PM -0400, Eric Chanudet wrote:
> Defer releasing the detached file-system when calling namespace_unlock()
> during a lazy umount to return faster.
> 
> When requesting MNT_DETACH, the caller does not expect the file-system
> to be shut down upon returning from the syscall. Calling
> synchronize_rcu_expedited() has a significant cost on RT kernel that
> defaults to rcupdate.rcu_normal_after_boot=1. Queue the detached struct
> mount in a separate list and put it on a workqueue to run post RCU
> grace-period.

For the past couple of days we've been seeing failures in a bunch of LTP
filesystem related tests on various arm64 systems.  The failures are
mostly (I think all) in the form:

20101 10:12:40.378045  tst_test.c:1833: TINFO: === Testing on vfat ===
20102 10:12:40.385091  tst_test.c:1170: TINFO: Formatting /dev/loop0 with vfat opts='' extra opts=''
20103 10:12:40.391032  mkfs.vfat: unable to open /dev/loop0: Device or resource busy
20104 10:12:40.395953  tst_test.c:1170: TBROK: mkfs.vfat failed with exit code 1

ie, a failure to stand up the test environment on the loopback device
all happening immediately after some other filesystem related test which
also used the loop device.  A bisect points to commit a6c7a78f1b6b97
which is this, which does look rather relevant.  LTP is obviously being
very much an edge case here.

Bisect log:

git bisect start
# status: waiting for both good and bad commits
# bad: [f660850bc246fef15ba78c81f686860324396628] Add linux-next specific files for 20250416
git bisect bad f660850bc246fef15ba78c81f686860324396628
# status: waiting for good commit(s), bad commit known
# good: [a6b9fbe391e8da36d2892590db4db4ff94005807] Merge branch 'for-linux-next-fixes' of https://gitlab.freedesktop.org/drm/misc/kernel.git
git bisect good a6b9fbe391e8da36d2892590db4db4ff94005807
# bad: [c017ce6f8d2939445ac473ada6a266aca0a0d6eb] Merge branch 'drm-next' of https://gitlab.freedesktop.org/drm/kernel.git
git bisect bad c017ce6f8d2939445ac473ada6a266aca0a0d6eb
# bad: [3efe6d22f422cbba9de75b53890c624a83dbb70a] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
git bisect bad 3efe6d22f422cbba9de75b53890c624a83dbb70a
# good: [ce44f781015a988baf21317f7822567a62a77a5f] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/qcom/linux.git
git bisect good ce44f781015a988baf21317f7822567a62a77a5f
# good: [64a47089f778b6e4bfaaf62d4384eaa2bcaf9b63] Merge branch 'overlayfs-next' of git://git.kernel.org/pub/scm/linux/kernel/git/overlayfs/vfs.git
git bisect good 64a47089f778b6e4bfaaf62d4384eaa2bcaf9b63
# good: [cdb4a05e60b2646d25f7227c7dfe5d54c3f3a173] Merge branch 'for-next' of git://github.com/openrisc/linux.git
git bisect good cdb4a05e60b2646d25f7227c7dfe5d54c3f3a173
# good: [00b7410736b1d46ab26c3b4e04eaa819e3f7448c] Merge branch 'vfs-6.16.misc' into vfs.all
git bisect good 00b7410736b1d46ab26c3b4e04eaa819e3f7448c
# bad: [a9d6e19f91b6600c02276cd7903f747a5389950c] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git
git bisect bad a9d6e19f91b6600c02276cd7903f747a5389950c
# bad: [03e1a90f178e3cea3e8864135046e31f4dbe5e2f] Merge branch 'vfs-6.16.mount' into vfs.all
git bisect bad 03e1a90f178e3cea3e8864135046e31f4dbe5e2f
# good: [a9d7de0f68b79e5e481967fc605698915a37ac13] Merge patch series "pidfs: ensure consistent ENOENT/ESRCH reporting"
git bisect good a9d7de0f68b79e5e481967fc605698915a37ac13
# bad: [675e87c588fc7d054c8f626fd59fcad6c534f4c0] selftests/mount_settattr: add missing STATX_MNT_ID_UNIQUE define
git bisect bad 675e87c588fc7d054c8f626fd59fcad6c534f4c0
# bad: [449f3214ce15b697277d5991f096140cf773e849] selftests/mount_settattr: don't define sys_open_tree() twice
git bisect bad 449f3214ce15b697277d5991f096140cf773e849
# bad: [a6c7a78f1b6b974a10fcf4646769ba8bf2596c58] fs/namespace: defer RCU sync for MNT_DETACH umount
git bisect bad a6c7a78f1b6b974a10fcf4646769ba8bf2596c58
# first bad commit: [a6c7a78f1b6b974a10fcf4646769ba8bf2596c58] fs/namespace: defer RCU sync for MNT_DETACH umount

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