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Message-ID: <24d4d60f-05f3-472b-8dfc-4edcb5f7883c@acm.org>
Date: Mon, 20 May 2024 11:11:52 -0700
From: Bart Van Assche <bvanassche@....org>
To: Yang Yang <yang.yang@...o.com>, Jens Axboe <axboe@...nel.dk>,
linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] blk-mq: fix potential I/O hang caused by batch wakeup
On 5/19/24 20:38, Yang Yang wrote:
> The depth is 62, and the wake_batch is 8. In the following situation,
> the task would hang forever.
>
> t1: t2: t3:
> blk_mq_get_tag . .
> io_schedule . .
> elevator_switch .
> blk_mq_freeze_queue .
> blk_freeze_queue_start .
> blk_mq_freeze_queue_wait .
> blk_mq_submit_bio
> __bio_queue_enter
>
> Fix this issue by waking up all the waiters sleeping on tags after
> freezing the queue.
Shouldn't blk_mq_alloc_request() be mentioned in t1 since that is the function
that calls blk_queue_enter()?
> diff --git a/block/blk-core.c b/block/blk-core.c
> index a16b5abdbbf5..e1eacfad6e5b 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -298,8 +298,6 @@ void blk_queue_start_drain(struct request_queue *q)
> * prevent I/O from crossing blk_queue_enter().
> */
> blk_freeze_queue_start(q);
> - if (queue_is_mq(q))
> - blk_mq_wake_waiters(q);
> /* Make blk_queue_enter() reexamine the DYING flag. */
> wake_up_all(&q->mq_freeze_wq);
> }
Why has blk_queue_start_drain() been modified? I don't see any reference
in the patch description to blk_queue_start_drain(). Am I perhaps missing
something?
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4ecb9db62337..9eb3139e713a 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -125,8 +125,10 @@ void blk_freeze_queue_start(struct request_queue *q)
> if (++q->mq_freeze_depth == 1) {
> percpu_ref_kill(&q->q_usage_counter);
> mutex_unlock(&q->mq_freeze_lock);
> - if (queue_is_mq(q))
> + if (queue_is_mq(q)) {
> + blk_mq_wake_waiters(q);
> blk_mq_run_hw_queues(q, false);
> + }
> } else {
> mutex_unlock(&q->mq_freeze_lock);
> }
Why would the above change be necessary? If the blk_queue_enter() call
by blk_mq_alloc_request() succeeds and blk_mq_get_tag() calls
io_schedule(), io_schedule() will be woken up indirectly by the
blk_mq_run_hw_queues() call because that call will free one of the tags
that the io_schedule() call is waiting for.
Thanks,
Bart.
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