[moderation] [fs?] KCSAN: data-race in d_make_discardable / lookup_fast

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c2f2b01b74be Merge tag 'i3c/for-6.19' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=124f4ec2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c3201432211be40f
dashboard link: https://syzkaller.appspot.com/bug?extid=7184882226c848d0286e
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
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/15a5e839a554/disk-c2f2b01b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/038ca01b3a1e/vmlinux-c2f2b01b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1b489de2e6a0/bzImage-c2f2b01b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+718488...@syzkaller.appspotmail.com

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

write to 0xffff8881185119c0 of 4 bytes by task 3965 on cpu 0:
d_make_discardable+0x4f/0xa0 fs/dcache.c:936
simple_unlink+0x68/0x80 fs/libfs.c:815
shmem_unlink+0x12d/0x140 mm/shmem.c:3991
vfs_unlink+0x28b/0x440 fs/namei.c:5369
do_unlinkat+0x1cd/0x4b0 fs/namei.c:5439
__do_sys_unlink fs/namei.c:5474 [inline]
__se_sys_unlink fs/namei.c:5472 [inline]
__x64_sys_unlink+0x2e/0x40 fs/namei.c:5472
x64_sys_call+0x2f48/0x3000 arch/x86/include/generated/asm/syscalls_64.h:88
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff8881185119c0 of 4 bytes by task 3004 on cpu 1:
d_revalidate fs/namei.c:968 [inline]
lookup_fast+0xf0/0x320 fs/namei.c:1806
walk_component fs/namei.c:2217 [inline]
lookup_last fs/namei.c:2722 [inline]
path_lookupat+0x15c/0x500 fs/namei.c:2746
filename_lookup+0x147/0x340 fs/namei.c:2775
do_readlinkat+0x7d/0x320 fs/stat.c:576
__do_sys_readlink fs/stat.c:613 [inline]
__se_sys_readlink fs/stat.c:610 [inline]
__x64_sys_readlink+0x47/0x60 fs/stat.c:610
x64_sys_call+0x2af1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:90
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x08300080 -> 0x00004080

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3004 Comm: udevd 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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