[moderation] [fs?] KCSAN: data-race in __d_instantiate / lookup_fast (12)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e84d960149e7 Merge tag 'for-6.19-rc5-tag' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=175fdb9a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=676c6f0212d0c041
dashboard link: https://syzkaller.appspot.com/bug?extid=1db8e7bac42204b93ef4
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/e2c3723b9ab1/disk-e84d9601.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5790bc964aa6/vmlinux-e84d9601.xz
kernel image: https://storage.googleapis.com/syzbot-assets/023ce82e3277/bzImage-e84d9601.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1db8e7...@syzkaller.appspotmail.com

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

read-write to 0xffff88811b936240 of 4 bytes by task 23522 on cpu 0:
fsnotify_update_flags include/linux/fsnotify_backend.h:-1 [inline]
__d_instantiate+0x2a8/0x390 fs/dcache.c:1977
d_make_persistent+0x78/0x110 fs/dcache.c:2793
shmem_mknod+0x15f/0x170 mm/shmem.c:3892
shmem_create+0x34/0x50 mm/shmem.c:3943
lookup_open fs/namei.c:4449 [inline]
open_last_lookups fs/namei.c:4549 [inline]
path_openat+0x1121/0x23c0 fs/namei.c:4793
do_filp_open+0x15d/0x280 fs/namei.c:4823
do_sys_openat2+0xa6/0x150 fs/open.c:1430
do_sys_open fs/open.c:1436 [inline]
__do_sys_open fs/open.c:1444 [inline]
__se_sys_open fs/open.c:1440 [inline]
__x64_sys_open+0xe6/0x110 fs/open.c:1440
x64_sys_call+0x166f/0x3000 arch/x86/include/generated/asm/syscalls_64.h:3
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b936240 of 4 bytes by task 23520 on cpu 1:
d_revalidate fs/namei.c:976 [inline]
lookup_fast+0xf0/0x320 fs/namei.c:1814
lookup_fast_for_open fs/namei.c:4486 [inline]
open_last_lookups fs/namei.c:4520 [inline]
path_openat+0x415/0x23c0 fs/namei.c:4793
do_filp_open+0x15d/0x280 fs/namei.c:4823
do_sys_openat2+0xa6/0x150 fs/open.c:1430
do_sys_open fs/open.c:1436 [inline]
__do_sys_open fs/open.c:1444 [inline]
__se_sys_open fs/open.c:1440 [inline]
__x64_sys_open+0xe6/0x110 fs/open.c:1440
x64_sys_call+0x166f/0x3000 arch/x86/include/generated/asm/syscalls_64.h:3
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000080 -> 0x08200080

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 23520 Comm: syz.8.4849 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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