Re: [PATCH v3] workqueue: mark pool->nr_running as __data_racy

[Breno Leitao <leitao@debian.org>]

On Wed, Oct 14, 2026 at 10:43:41PM +0700, Nguyen Ngoc Thang wrote:
> wq_worker_running() increments pool->nr_running with only preemption
> disabled, while kick_pool_pick() reads it via need_more_worker() under
> pool->lock, so the two can run concurrently on different CPUs. KCSAN
> reports:
> 
>   BUG: KCSAN: data-race in kick_pool_pick / wq_worker_running
> 
>   write to 0xffff888059c2af24 of 4 bytes by task 3007 on cpu 0:
>    wq_worker_running+0x114/0x130
>    schedule_timeout+0x16e/0x180
>    io_schedule_timeout+0x4c/0x70
>    submit_bio_wait+0xd7/0x110
>    blkdev_issue_flush+0x4c/0x70
>    ext4_sync_file+0x60d/0x6c0
>    aio_fsync_work+0x7f/0x270
>    process_one_work+0x4ae/0xa00
> 
>   read to 0xffff888059c2af24 of 4 bytes by task 3001 on cpu 1:
>    kick_pool_pick+0x7a/0x3a0
>    __queue_work+0x72d/0xcf0
>    queue_work_on+0x9d/0xb0
>    schedule_on_each_cpu+0x11a/0x2d0
>    vmstat_refresh+0x23/0x140
>    proc_sys_call_handler+0x2b9/0x4f0
> 
> The race is benign. The increment in wq_worker_running() is the only
> lockless write to nr_running; decrements and the reset are done under
> pool->lock, so a reader holding the lock sees either the old or the new
> count. A stale 0 costs at most a spurious wakeup of an idle worker. A
> nonzero count is correct because the incrementing worker is running and
> re-checks the worklist under pool->lock before it stops counting. The
> increment can also go 1 -> 2, e.g. when a worker sleeping in a work item
> wakes up after kick_pool() woke another worker.
> 
> The lockless increment is part of how nr_running is designed to work,
> as the comment above the field documents. Mark the field __data_racy
> so that every reader is covered, not just need_more_worker().
> 
> Reported-by: syzbot+8922e6b8343f70be92e8@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=8922e6b8343f70be92e8
> Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>

Reviewed-by: Breno Leitao <leitao@debian.org>