[moderation] [fs?] KCSAN: data-race in lookup_fast / shrink_dentry_list (13)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 23b0f90ba871 Merge tag 'sysctl-7.00-rc1' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11303b3a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=70c3ed59b49365c3
dashboard link: https://syzkaller.appspot.com/bug?extid=21eda88da7eb80523311
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/ae3bcc875a04/disk-23b0f90b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1a4dbd53da8f/vmlinux-23b0f90b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3c3824777f5c/bzImage-23b0f90b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+21eda8...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in lookup_fast / shrink_dentry_list

read-write to 0xffff888187eb4f00 of 4 bytes by task 23855 on cpu 1:
d_shrink_del fs/dcache.c:516 [inline]
shrink_dentry_list+0x22d/0x3c0 fs/dcache.c:1166
shrink_dcache_sb+0x15d/0x290 fs/dcache.c:1295
reconfigure_super+0x142/0x590 fs/super.c:1058
do_remount fs/namespace.c:3388 [inline]
path_mount+0xaf7/0xbc0 fs/namespace.c:4138
do_mount fs/namespace.c:4159 [inline]
__do_sys_mount fs/namespace.c:4348 [inline]
__se_sys_mount+0x28c/0x2e0 fs/namespace.c:4325
__x64_sys_mount+0x67/0x80 fs/namespace.c:4325
x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888187eb4f00 of 4 bytes by task 23415 on cpu 0:
d_revalidate fs/namei.c:1029 [inline]
lookup_fast+0x72/0x320 fs/namei.c:1877
walk_component fs/namei.c:2275 [inline]
link_path_walk+0x51c/0xe30 fs/namei.c:2647
path_lookupat+0x8c/0x500 fs/namei.c:2803
filename_lookup+0x190/0x390 fs/namei.c:2833
user_path_at+0x3e/0x100 fs/namei.c:3612
ksys_umount fs/namespace.c:2052 [inline]
__do_sys_umount fs/namespace.c:2060 [inline]
__se_sys_umount fs/namespace.c:2058 [inline]
__x64_sys_umount+0x85/0xe0 fs/namespace.c:2058
x64_sys_call+0x103b/0x3020 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00140444 -> 0x00100044

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 23415 Comm: syz-executor Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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