[moderation] [ext4?] KCSAN: data-race in wb_wait_for_completion / writeback_sb_inodes

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2061f18ad76e Merge tag 'caps-pr-20251204' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=124fb192580000
kernel config: https://syzkaller.appspot.com/x/.config?x=fe265072cbd2f67a
dashboard link: https://syzkaller.appspot.com/bug?extid=a5ba3c3cc12b79566903
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [adilger...@dilger.ca linux...@vger.kernel.org linux-...@vger.kernel.org ty...@mit.edu]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/68ad9eabb708/disk-2061f18a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/756a6923eb62/vmlinux-2061f18a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/dbf0c80cc61c/bzImage-2061f18a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a5ba3c...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in wb_wait_for_completion / writeback_sb_inodes

write to 0xffffc90001567cb8 of 8 bytes by task 3318 on cpu 0:
wb_wait_for_completion_cb fs/fs-writeback.c:203 [inline]
wb_wait_for_completion+0x90/0x360 fs/fs-writeback.c:226
__writeback_inodes_sb_nr fs/fs-writeback.c:2808 [inline]
writeback_inodes_sb_nr fs/fs-writeback.c:2825 [inline]
writeback_inodes_sb+0x144/0x190 fs/fs-writeback.c:2840
sync_filesystem+0x69/0x190 fs/sync.c:54
generic_shutdown_super+0x44/0x210 fs/super.c:622
kill_block_super+0x2a/0x70 fs/super.c:1730
ext4_kill_sb+0x42/0x80 fs/ext4/super.c:7444
deactivate_locked_super+0x75/0x1c0 fs/super.c:474
deactivate_super+0x97/0xa0 fs/super.c:507
cleanup_mnt+0x2a9/0x320 fs/namespace.c:1318
__cleanup_mnt+0x19/0x20 fs/namespace.c:1325
task_work_run+0x131/0x1a0 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:44 [inline]
exit_to_user_mode_loop+0x1fe/0x740 kernel/entry/common.c:75
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
do_syscall_64+0x202/0x2a0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffc90001567cb8 of 8 bytes by task 6444 on cpu 1:
writeback_sb_inodes+0x4ce/0xb20 fs/fs-writeback.c:2033
wb_writeback+0x252/0x5c0 fs/fs-writeback.c:2216
wb_do_writeback fs/fs-writeback.c:2363 [inline]
wb_workfn+0x194/0x910 fs/fs-writeback.c:2403
process_one_work kernel/workqueue.c:3257 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
worker_thread+0x582/0x770 kernel/workqueue.c:3421
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

value changed: 0x0000000000000000 -> 0x00000000ffffb2c0

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 6444 Comm: kworker/u8:34 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Workqueue: writeback wb_workfn (flush-7:1)
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup