On 6/7/21 5:26 AM, Jan Kara wrote:
> Commit 545fbd0775ba ("rq-qos: fix missed wake-ups in rq_qos_throttle")
> tried to fix a problem that a process could be sleeping in rq_qos_wait()
> without anyone to wake it up. However the fix is not complete and the
> following can still happen:
>
> CPU1 (waiter1) CPU2 (waiter2) CPU3 (waker)
> rq_qos_wait() rq_qos_wait()
> acquire_inflight_cb() -> fails
> acquire_inflight_cb() -> fails
>
> completes IOs, inflight
> decreased
> prepare_to_wait_exclusive()
> prepare_to_wait_exclusive()
> has_sleeper = !wq_has_single_sleeper() -> true as there are two sleepers
> has_sleeper = !wq_has_single_sleeper() -> true
> io_schedule() io_schedule()
>
> Deadlock as now there's nobody to wakeup the two waiters. The logic
> automatically blocking when there are already sleepers is really subtle
> and the only way to make it work reliably is that we check whether there
> are some waiters in the queue when adding ourselves there. That way, we
> are guaranteed that at least the first process to enter the wait queue
> will recheck the waiting condition before going to sleep and thus
> guarantee forward progress.
Applied, thanks.
--
Jens Axboe
