linux-ext4 - [Bug 219283] kernel regression with ext4 and ea_inode mount flags and exercising xattrs (between Linux 6.8 and 6.11)

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Message-ID: <bug-219283-13602-DlXoU1fag7@https.bugzilla.kernel.org/>
Date: Wed, 18 Sep 2024 16:08:33 +0000
From: bugzilla-daemon@...nel.org
To: linux-ext4@...r.kernel.org
Subject: [Bug 219283] kernel regression with ext4 and ea_inode mount flags
 and exercising xattrs (between Linux 6.8 and 6.11)

https://bugzilla.kernel.org/show_bug.cgi?id=219283

--- Comment #11 from Jan Kara (jack@...e.cz) ---
I think I see the problem and AFAICT commit 0a46ef234756dca is just making the
latent deadlock easier to hit. The problem is shown by these two stacktraces
from your dmesg:

Task 1
[  247.045575]  __wait_on_freeing_inode+0xba/0x140
[  247.045584]  find_inode_fast+0xa4/0xe0
[  247.045588]  iget_locked+0x71/0x200
[  247.045597]  __ext4_iget+0x148/0x1080
[  247.045615]  ext4_xattr_inode_cache_find+0xe2/0x220
[  247.045621]  ext4_xattr_inode_lookup_create+0x122/0x240
[  247.045626]  ext4_xattr_block_set+0xc2/0xeb0
[  247.045633]  ext4_xattr_set_handle+0x4ba/0x650
[  247.045641]  ext4_xattr_set+0x80/0x160

Task 2
[  247.043719]  mb_cache_entry_wait_unused+0x9a/0xd0
[  247.043729]  ext4_evict_ea_inode+0x64/0xb0
[  247.043733]  ext4_evict_inode+0x35c/0x6d0
[  247.043739]  evict+0x108/0x2c0
[  247.043745]  iput+0x14a/0x260
[  247.043749]  ext4_xattr_ibody_set+0x175/0x1d0
[  247.043754]  ext4_xattr_set_handle+0x297/0x650
[  247.043762]  ext4_xattr_set+0x80/0x160

These two tasks are deadlocked against each other. One has dropped the last
reference to xattr inode and is trying to remove it from memory and waits for
corresponding mbcache entry to get unused while another task is holding the
mbcache entry reference and is waiting for inode to be evicted from memory.
Commit 0a46ef234756dca removed synchronization on buffer lock for one of the
hot paths and thus hitting this race is now much more likely.

I just have to make up my mind how to best fix this ABBA deadlock.

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