Re: [BUG REPORT] shrink_dcache_parent() loops indefinitely on a next-20240102 kernel

[Chandan Babu R <chandanbabu@kernel.org>]

On Thu, Jan 04, 2024 at 06:40:43 PM +0530, Chandan Babu R wrote:
> On Wed, Jan 03, 2024 at 08:34:20 PM -0800, Darrick J. Wong wrote:
>> On Wed, Jan 03, 2024 at 12:12:12PM +0530, Chandan Babu R wrote:
>>> Hi,
>>> 
>>> Executing fstests' recoveryloop test group on XFS on a next-20240102 kernel
>>> sometimes causes the following hung task report to be printed on the console,
>>> 

Meanwhile, I have executed some more experiments.

The bug can be recreated on a next-20240102 kernel by executing either
generic/388 or generic/475 for a maximum of 10 iterations. I tried to do a git
bisect based on this observation i.e. I would mark a commit as 'good' if the
bug does not get recreated within 10 iterations. This led to the following git
bisect log,

# git bisect log
# bad: [ab0b3e6ef50d305278b1971891cf1d82ab050b35] Add linux-next specific files for 20240102
# good: [33cc938e65a98f1d29d0a18403dbbee050dcad9a] Linux 6.7-rc4
git bisect start 'HEAD' 'v6.7-rc4' 'fs/'
# bad: [ca20194665a58bb541cc9e4dc7abcf96a7c96bd9] Merge branch 'main' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git
git bisect bad ca20194665a58bb541cc9e4dc7abcf96a7c96bd9
# good: [bb986dac9a56f88552418288c87e2223f8f448e3] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git
git bisect good bb986dac9a56f88552418288c87e2223f8f448e3
# bad: [964c80149d24b33bbddd222edbcc782be6eab841] Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git
git bisect bad 964c80149d24b33bbddd222edbcc782be6eab841
# good: [da9e5cca7e753dd96e07ae9212f4aeec1b9c68a6] Merge branch 'vfs.all' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
git bisect good da9e5cca7e753dd96e07ae9212f4aeec1b9c68a6
# bad: [9e02829aa9d2e5f2e080ba0191c36a50a384acf1] Merge branch 'hwmon-next' of git://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging.git
git bisect bad 9e02829aa9d2e5f2e080ba0191c36a50a384acf1
# bad: [3ac3331ea2c1826e3d30276e0a7e7e62fff519a8] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
git bisect bad 3ac3331ea2c1826e3d30276e0a7e7e62fff519a8
# bad: [e01e3fb272cf7380dd5d70f52e955ac1122e2184] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git
git bisect bad e01e3fb272cf7380dd5d70f52e955ac1122e2184
# bad: [6d06b73bcd6eee6ca43f429a28e812ef2ad7a4ea] Merge branch 'work.simple_recursive_removal' into for-next
git bisect bad 6d06b73bcd6eee6ca43f429a28e812ef2ad7a4ea
# bad: [119dcc73a9c2df0da002054cdb2296cb32b7cb93] Merge branches 'work.dcache-misc' and 'work.dcache2' into work.dcache
git bisect bad 119dcc73a9c2df0da002054cdb2296cb32b7cb93
# good: [6367b491c17a34b28aece294bddfda1a36ec0377] retain_dentry(): introduce a trimmed-down lockless variant
git bisect good 6367b491c17a34b28aece294bddfda1a36ec0377
# good: [b33c14c8618edfc00bf8963e3b0c8a2b19c9eaa4] Merge branch 'no-rebase-overlayfs' into work.dcache-misc
git bisect good b33c14c8618edfc00bf8963e3b0c8a2b19c9eaa4
# good: [f9453a1ad1fadae29fd7db5ad8ea16f35e737276] Merge branch 'merged-selinux' into work.dcache-misc
git bisect good f9453a1ad1fadae29fd7db5ad8ea16f35e737276
# good: [57851607326a2beef21e67f83f4f53a90df8445a] get rid of DCACHE_GENOCIDE
git bisect good 57851607326a2beef21e67f83f4f53a90df8445a
# good: [ef69f0506d8f3a250ac5baa96746e17ae22c67b5] __d_unalias() doesn't use inode argument
git bisect good ef69f0506d8f3a250ac5baa96746e17ae22c67b5
# first bad commit: [119dcc73a9c2df0da002054cdb2296cb32b7cb93] Merge branches 'work.dcache-misc' and 'work.dcache2' into work.dcache

Looks like the bug is caused by changes that were made in two separate
branches.

I have also confirmed that the bug was not present in the v6.7-rc4 kernel by
executing generic/388 in a loop for 44 times.

I can also confirm that the issue can be recreated on a next-20240118 kernel.

-- 
Chandan