KCSAN: data-race in __d_instantiate / lookup_fast (14)

Status: moderation: reported on 2026/08/21 13:23
Subsystems: fs
Labels: race:benign
[Documentation on labels]
Reported-by: syzbot+4b5d33c94277a9f91461@syzkaller.appspotmail.com
First crash: 1d07h, last: 1d07h
▶ ▼ ✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision
ac118b6d assessment-kcsan Benign: ✅ ❓ KCSAN: data-race in __d_instantiate / lookup_fast (14) 2026/08/21 13:22 2026/08/21 13:23 2026/08/21 13:57 1e72964b
▶ ▼ Similar bugs (13)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream KCSAN: data-race in __d_instantiate / lookup_fast (12) fs 6 4 177d 215d 0/29 auto-obsoleted due to no activity on 2026/04/22 20:55
upstream KCSAN: data-race in __d_instantiate / lookup_fast (6) ext4 6 53 831d 987d 0/29 auto-obsoleted due to no activity on 2024/06/16 23:52
upstream KCSAN: data-race in __d_instantiate / lookup_fast (13) race:benign prio:low fs 6 1 85d 85d 0/29 auto-obsoleted due to no activity on 2026/07/24 11:02
upstream KCSAN: data-race in __d_instantiate / lookup_fast ext4 6 6 2309d 2433d 0/29 auto-closed as invalid on 2020/06/18 13:33
upstream KCSAN: data-race in __d_instantiate / lookup_fast (4) ext4 6 16 1477d 1592d 0/29 auto-closed as invalid on 2022/09/10 18:37
upstream KCSAN: data-race in __d_instantiate / lookup_fast (5) ext4 6 7 1385d 1438d 0/29 auto-obsoleted due to no activity on 2022/12/11 12:03
upstream KCSAN: data-race in __d_instantiate / lookup_fast (2) fs 6 1 2018d 2018d 0/29 auto-closed as invalid on 2021/03/18 02:49
upstream KCSAN: data-race in __d_instantiate / lookup_fast (3) ext4 6 1 1865d 1865d 0/29 auto-closed as invalid on 2021/08/17 22:04
upstream KCSAN: data-race in __d_instantiate / lookup_fast (10) fs 6 1 450d 450d 0/29 auto-obsoleted due to no activity on 2025/07/24 00:58
upstream KCSAN: data-race in __d_instantiate / lookup_fast (8) fs 6 1 684d 684d 0/29 auto-obsoleted due to no activity on 2024/11/11 02:07
upstream KCSAN: data-race in __d_instantiate / lookup_fast (9) fs 6 1 593d 593d 0/29 auto-obsoleted due to no activity on 2025/03/03 11:43
upstream KCSAN: data-race in __d_instantiate / lookup_fast (7) ext4 6 1 793d 793d 0/29 auto-obsoleted due to no activity on 2024/07/25 04:38
upstream KCSAN: data-race in __d_instantiate / lookup_fast (11) fs 6 1 358d 358d 0/29 auto-obsoleted due to no activity on 2025/10/24 20:01

Sample crash report:
==================================================================
BUG: KCSAN: data-race in __d_instantiate / lookup_fast

read-write to 0xffff88810049b180 of 4 bytes by task 24946 on cpu 1:
 fsnotify_update_flags include/linux/fsnotify_backend.h:-1 [inline]
 __d_instantiate+0x2a8/0x390 fs/dcache.c:2154
 d_make_persistent+0x72/0x110 fs/dcache.c:2978
 shmem_symlink+0x3eb/0x400 mm/shmem.c:4084
 vfs_symlink+0xfd/0x220 fs/namei.c:5794
 filename_symlinkat+0xe8/0x2b0 fs/namei.c:5819
 __do_sys_symlink fs/namei.c:5846 [inline]
 __se_sys_symlink+0x42/0x1b0 fs/namei.c:5842
 __x64_sys_symlink+0x31/0x40 fs/namei.c:5842
 x64_sys_call+0x211f/0x2550 arch/x86/include/generated/asm/syscalls_64.h:89
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810049b180 of 4 bytes by task 2980 on cpu 0:
 d_revalidate fs/namei.c:1034 [inline]
 lookup_fast+0xf1/0x320 fs/namei.c:1868
 walk_component fs/namei.c:2278 [inline]
 lookup_last fs/namei.c:2789 [inline]
 path_lookupat+0x120/0x4c0 fs/namei.c:2813
 filename_lookup+0x18f/0x390 fs/namei.c:2842
 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+0x1d41/0x2550 arch/x86/include/generated/asm/syscalls_64.h:90
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000080 -> 0x08300080

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 2980 Comm: udevd Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/21 13:22 upstream 818bebeb63dd 1e72964b .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in __d_instantiate / lookup_fast
* Struck through repros no longer work on HEAD.