inotify watch not triggered with PREEMPT_RT=y on KVM

inotify watch not triggered with PREEMPT_RT=y on KVM

[Varad Gautam]

Hi,

I'm building coreutils on linux-rt-devel v5.12-rc3-rt3 e164cb00df1 [1]
running on KVM, and I see a consistent failure in one of the coreutils
`tail` testcases [2]:

FAIL tests/tail-2/assert.sh (exit status: 1)

I see that on deleting a file, the inode gets dropped, but the inotify
watch that tail registered on it sometimes does not get notified. This
is unexpected for tail without --retry.

# touch foo; tail --follow=name foo & pid=$!

# cat /proc/$pid/fdinfo/4 | grep inotify
inotify wd:2 ino:123a2 sdev:fe00000 mask:c06 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:a22301004fb29f63
inotify wd:1 ino:1239f sdev:fe00000 mask:784 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:9f23010072246970

# ls -i
74658 foo

# find . -inum 74658
./foo

# rm foo

# find . -inum 74658

# cat /proc/$pid/fdinfo/4 | grep inotify
inotify wd:2 ino:123a2 sdev:fe00000 mask:c06 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:a22301004fb29f63
inotify wd:1 ino:1239f sdev:fe00000 mask:784 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:9f23010072246970

I can also reproduce this on linux-stable-rt v5.4-rt-rebase (0dc167c2b2) [3]
running as a KVM guest. Non-PREEMPT_RT builds work fine, and so does PREEMPT_RT
when running on bare-metal.

Any suggestions on what might be happening here?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?id=e164cb00df1c
[2] https://github.com/coreutils/coreutils/blob/master/tests/tail-2/assert.sh
[3] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/commit/?h=v5.4-rt-rebase&id=0dc167c2b2

Regards,
Varad

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