[moderation] [fs?] KCSAN: data-race in dentry_kill / fast_dput

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 95e56f0f293e Merge tag 'kcsan-20260612-v7.2-rc1' of git://..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=121a84ae580000
kernel config: https://syzkaller.appspot.com/x/.config?x=db037fe04c31f5d8
dashboard link: https://syzkaller.appspot.com/bug?extid=1cfd29c242c7cd26da02
compiler: Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/aa890212e11a/disk-95e56f0f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/db4244fdd907/vmlinux-95e56f0f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/410919fa3f7d/bzImage-95e56f0f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1cfd29...@syzkaller.appspotmail.com

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

write to 0xffff88811bf91e50 of 8 bytes by task 2982 on cpu 1:
__d_drop fs/dcache.c:578 [inline]
dentry_kill+0x139/0x500 fs/dcache.c:824
finish_dput+0x16/0x1d0 fs/dcache.c:1001
dput+0x48/0x50 fs/dcache.c:1042
handle_mounts fs/namei.c:1753 [inline]
step_into_slowpath+0x38c/0x4c0 fs/namei.c:2104
step_into fs/namei.c:2152 [inline]
walk_component fs/namei.c:2288 [inline]
lookup_last fs/namei.c:2789 [inline]
path_lookupat+0x2eb/0x4c0 fs/namei.c:2813
filename_lookup+0x329/0x390 fs/namei.c:2844
do_readlinkat+0x74/0x2f0 fs/stat.c:571
__do_sys_readlink fs/stat.c:605 [inline]
__se_sys_readlink fs/stat.c:602 [inline]
__x64_sys_readlink+0x47/0x60 fs/stat.c:602
x64_sys_call+0x2b51/0x3020 arch/x86/include/generated/asm/syscalls_64.h:90
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811bf91e50 of 8 bytes by task 18264 on cpu 0:
hlist_bl_unhashed include/linux/list_bl.h:54 [inline]
d_unhashed include/linux/dcache.h:396 [inline]
retain_dentry fs/dcache.c:872 [inline]
fast_dput+0x64/0x2e0 fs/dcache.c:971
dput+0x1f/0x50 fs/dcache.c:1040
end_dirop fs/namei.c:2956 [inline]
filename_unlinkat+0x217/0x410 fs/namei.c:5584
__do_sys_unlink fs/namei.c:5616 [inline]
__se_sys_unlink+0x2b/0xe0 fs/namei.c:5613
__x64_sys_unlink+0x1f/0x30 fs/namei.c:5613
x64_sys_call+0x2eb6/0x3020 arch/x86/include/generated/asm/syscalls_64.h:88
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff8882377e6140 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 18264 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================


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