[moderation] [ext4?] KCSAN: data-race in __d_rehash / fast_dput

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f2e367d6ad3b Merge tag 'for-6.8/dm-fix-3' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=176ab074180000
kernel config: https://syzkaller.appspot.com/x/.config?x=7507056e4ee79a
dashboard link: https://syzkaller.appspot.com/bug?extid=809dac4b9cd20451b967
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
CC: [adilger...@dilger.ca linux...@vger.kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org ty...@mit.edu]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/fe6008567e0e/disk-f2e367d6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/dcf8ebbdec59/vmlinux-f2e367d6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/516542631a41/bzImage-f2e367d6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+809dac...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __d_rehash / fast_dput

write to 0xffff8881496e9790 of 8 bytes by task 15097 on cpu 1:
hlist_bl_add_head_rcu include/linux/rculist_bl.h:81 [inline]
__d_rehash+0xc4/0x220 fs/dcache.c:2403
__d_add+0x36d/0x4a0 fs/dcache.c:2618
d_splice_alias+0xd6/0x270 fs/dcache.c:3009
ext4_lookup+0x199/0x390 fs/ext4/namei.c:1883
lookup_one_qstr_excl+0xc0/0x180 fs/namei.c:1608
filename_create+0x14d/0x2b0 fs/namei.c:3892
do_symlinkat+0x84/0x340 fs/namei.c:4499
__do_sys_symlinkat fs/namei.c:4522 [inline]
__se_sys_symlinkat fs/namei.c:4519 [inline]
__x64_sys_symlinkat+0x62/0x70 fs/namei.c:4519
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b

read to 0xffff8881496e9790 of 8 bytes by task 2773 on cpu 0:
hlist_bl_unhashed include/linux/list_bl.h:54 [inline]
d_unhashed include/linux/dcache.h:341 [inline]
retain_dentry fs/dcache.c:686 [inline]
fast_dput+0xcf/0x2b0 fs/dcache.c:799
dput+0x24/0xc0 fs/dcache.c:839
do_unlinkat+0x257/0x4c0 fs/namei.c:4401
__do_sys_unlink fs/namei.c:4446 [inline]
__se_sys_unlink fs/namei.c:4444 [inline]
__x64_sys_unlink+0x30/0x40 fs/namei.c:4444
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b

value changed: 0xffff888237431480 -> 0xffff888153601248

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 2773 Comm: udevd Tainted: G W 6.8.0-rc5-syzkaller-00297-gf2e367d6ad3b #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
==================================================================


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