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From: Ming Lei <ming.lei@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Hannes Reinecke <hare@suse.de>,
	John Garry <john.garry@huawei.com>,
	David Jeffery <djeffery@redhat.com>
Subject: Re: [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool
Date: Fri, 7 May 2021 14:31:02 +0800	[thread overview]
Message-ID: <YJTepqtgw3+K5vWr@T590> (raw)
In-Reply-To: <a2a7e66b-68e8-6468-2add-19ffbe096ed9@acm.org>

On Thu, May 06, 2021 at 08:16:25PM -0700, Bart Van Assche wrote:
> On 5/6/21 7:05 PM, Ming Lei wrote:
> > Putting the lock pair after clearing rq mapping should work, but not see
> > any benefit: not very readable, and memory barrier knowledge is required for
> > understanding its correctness(cmpxchg has to be completed before unlock), ...,
> > so is it better idea to move lock pair after clearing rq mapping?
> 
> It depends on how much time will be spent inside
> blk_mq_clear_rq_mapping(). If the time spent in the nested loop in
> blk_mq_clear_rq_mapping() would be significant then the proposed change
> will help to reduce interrupt latency in blk_mq_find_and_get_req().

interrupt latency in blk_mq_find_and_get_req() shouldn't be increased
because interrupt won't be disabled when spinning on the lock. But interrupt
may be disabled for a while in blk_mq_clear_rq_mapping() in case of big
nr_requests and hw queue depth.

Fair enough, will take this way for not holding lock for too long.

Thanks, 
Ming


  reply	other threads:[~2021-05-07  6:31 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05 14:58 [PATCH V5 0/4] blk-mq: fix request UAF related with iterating over tagset requests Ming Lei
2021-05-05 14:58 ` [PATCH V5 1/4] block: avoid double io accounting for flush request Ming Lei
2021-05-06  6:44   ` Christoph Hellwig
2021-05-05 14:58 ` [PATCH V5 2/4] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter Ming Lei
2021-05-06  6:54   ` Christoph Hellwig
2021-05-06  7:30     ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool Ming Lei
2021-05-06  7:12   ` Christoph Hellwig
2021-05-06  7:34     ` Ming Lei
2021-05-06 12:18       ` Christoph Hellwig
2021-05-06 13:38         ` Ming Lei
2021-05-07  6:57           ` Christoph Hellwig
2021-05-07  7:30             ` Ming Lei
2021-05-06 14:51         ` Bart Van Assche
2021-05-07  0:11           ` Ming Lei
2021-05-07  1:10             ` Bart Van Assche
2021-05-07  2:05               ` Ming Lei
2021-05-07  3:16                 ` Bart Van Assche
2021-05-07  6:31                   ` Ming Lei [this message]
2021-05-07  6:52                     ` Christoph Hellwig
2021-05-08  2:02                       ` Bart Van Assche
2021-05-06 15:02   ` Bart Van Assche
2021-05-07  0:13     ` Ming Lei
2021-05-07  1:11   ` Bart Van Assche
2021-05-07  2:06     ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 4/4] blk-mq: clearing flush request reference in tags->rqs[] Ming Lei

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