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Wed, 04 Sep 2024 09:04:43 -0400 X-MC-Unique: 8quKOHEAMIa7bzdo6jPLpQ-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D4ACF19560AE; Wed, 4 Sep 2024 13:04:41 +0000 (UTC) Received: from fedora (unknown [10.72.116.59]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 95DF3195605A; Wed, 4 Sep 2024 13:04:36 +0000 (UTC) Date: Wed, 4 Sep 2024 21:04:30 +0800 From: Ming Lei To: Muchun Song Cc: axboe@kernel.dk, yukuai1@huaweicloud.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, stable@vger.kernel.org Subject: Re: [PATCH v2 3/3] block: fix ordering between checking BLK_MQ_S_STOPPED and adding requests Message-ID: References: <20240903081653.65613-1-songmuchun@bytedance.com> <20240903081653.65613-4-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240903081653.65613-4-songmuchun@bytedance.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 On Tue, Sep 03, 2024 at 04:16:53PM +0800, Muchun Song wrote: > Supposing first scenario with a virtio_blk driver. > > CPU0 CPU1 > > blk_mq_try_issue_directly() > __blk_mq_issue_directly() > q->mq_ops->queue_rq() > virtio_queue_rq() > blk_mq_stop_hw_queue() > virtblk_done() > blk_mq_request_bypass_insert() blk_mq_start_stopped_hw_queues() > /* Add IO request to dispatch list */ 1) store blk_mq_start_stopped_hw_queue() > clear_bit(BLK_MQ_S_STOPPED) 3) store > blk_mq_run_hw_queue() blk_mq_run_hw_queue() > if (!blk_mq_hctx_has_pending()) if (!blk_mq_hctx_has_pending()) 4) load > return return > blk_mq_sched_dispatch_requests() blk_mq_sched_dispatch_requests() > if (blk_mq_hctx_stopped()) 2) load if (blk_mq_hctx_stopped()) > return return > __blk_mq_sched_dispatch_requests() __blk_mq_sched_dispatch_requests() > > Supposing another scenario. > > CPU0 CPU1 > > blk_mq_requeue_work() > /* Add IO request to dispatch list */ 1) store virtblk_done() > blk_mq_run_hw_queues()/blk_mq_delay_run_hw_queues() blk_mq_start_stopped_hw_queues() > if (blk_mq_hctx_stopped()) 2) load blk_mq_start_stopped_hw_queue() > continue clear_bit(BLK_MQ_S_STOPPED) 3) store > blk_mq_run_hw_queue()/blk_mq_delay_run_hw_queue() blk_mq_run_hw_queue() > if (!blk_mq_hctx_has_pending()) 4) load > return > blk_mq_sched_dispatch_requests() > > Both scenarios are similar, the full memory barrier should be inserted between > 1) and 2), as well as between 3) and 4) to make sure that either CPU0 sees > BLK_MQ_S_STOPPED is cleared or CPU1 sees dispatch list. Otherwise, either CPU > will not rerun the hardware queue causing starvation of the request. > > The easy way to fix it is to add the essential full memory barrier into helper > of blk_mq_hctx_stopped(). In order to not affect the fast path (hardware queue > is not stopped most of the time), we only insert the barrier into the slow path. > Actually, only slow path needs to care about missing of dispatching the request > to the low-level device driver. > > Fixes: 320ae51feed5c ("blk-mq: new multi-queue block IO queueing mechanism") > Cc: stable@vger.kernel.org > Cc: Muchun Song > Signed-off-by: Muchun Song Looks fine, Reviewed-by: Ming Lei Thanks, Ming