Re: [PATCH] rq-qos: fix missed wake-ups in rq_qos_throttle try two

From: Jens Axboe <axboe@kernel.dk>
To: Jan Kara <jack@suse.cz>
Cc: Josef Bacik <josef@toxicpanda.com>,
	Oleg Nesterov <oleg@redhat.com>,
	linux-block@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] rq-qos: fix missed wake-ups in rq_qos_throttle try two
Date: Tue, 8 Jun 2021 15:13:16 -0600	[thread overview]
Message-ID: <e14aeaa7-45a3-b2f0-7738-3613189ae1d4@kernel.dk> (raw)
In-Reply-To: <20210607112613.25344-1-jack@suse.cz>

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