Re: [PATCH] io_uring/io-wq: avoid repeated task_work scans during teardown

Hi Peter & Steven:
Do you have time to help review this patch ?

Thanks.


> From: "Jens Axboe"<[email protected]>
> Date:  Fri, May 22, 2026, 00:59
> Subject:  Re: [PATCH] io_uring/io-wq: avoid repeated task_work scans during teardown
> To: "Fengnan Chang"<[email protected]>, <[email protected]>, <[email protected]>, <[email protected]>, <[email protected]>
> On 5/19/26 9:12 PM, Fengnan Chang wrote:
> > We hit hard-lockup reports from iou-wrk threads stuck in
> > task_work_cancel_match() during io-wq teardown in syzkaller test.
> > The root cause is that teardown repeatedly rescans the submitter task's
> > full task_work list under pi_lock, once per matched item.
> > 
> > Two spots are problematic:
> > 
> > 1) io_wq_cancel_tw_create() loops calling task_work_cancel_match() to
> >    remove worker-creation callbacks one at a time. Each call re-walks
> >    the entire list from scratch while holding pi_lock.
> > 
> > 2) io_worker_exit() unconditionally scans the submitter task_work list
> >    for its own create_work, even when it never queued one. With many
> >    workers exiting simultaneously against a large unrelated task_work
> >    list, this adds up fast.
> > 
> > Fix (1) by adding task_work_cancel_match_all() that unlinks all matching
> > callbacks in a single traversal, then iterating the returned list locally.
> > Same try_cmpxchg() synchronisation as before, stops at the work_exited
> > sentinel.
> > 
> > Fix (2) by skipping the cancel entirely unless create_state indicates a
> > pending create_work. Since create_state is exclusively owned via
> > test_and_set_bit_lock, at most one callback can be queued per worker, so
> > the cancel is also simplified from a loop to a single call.
> > 
> > With this fix the reproducer (FIFO-open + MSG_RING SEND_FD stress) no
> > longer triggers hard-lockup reports, and task_work_cancel_match samples
> > drop to microseconds.
> 
> Looks good to me, nicer way to do this too.
> 
> -- 
> Jens Axboe
>