From: Bart Van Assche <bart.vanassche@wdc.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: "hch@infradead.org" <hch@infradead.org>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
"snitzer@redhat.com" <snitzer@redhat.com>,
"axboe@kernel.dk" <axboe@kernel.dk>,
"dm-devel@redhat.com" <dm-devel@redhat.com>,
"jejb@linux.vnet.ibm.com" <jejb@linux.vnet.ibm.com>,
"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
"martin.petersen@oracle.com" <martin.petersen@oracle.com>,
"loberman@redhat.com" <loberman@redhat.com>
Subject: Re: [PATCH 1/5] blk-mq: introduce BLK_STS_DEV_RESOURCE
Date: Tue, 23 Jan 2018 08:37:26 -0800 [thread overview]
Message-ID: <4cdb2b42-0097-c64a-d9c0-b7a1727c80de@wdc.com> (raw)
In-Reply-To: <20180123162605.GD13277@ming.t460p>
On 01/23/18 08:26, Ming Lei wrote:
> On Tue, Jan 23, 2018 at 08:17:02AM -0800, Bart Van Assche wrote:
>> On 01/22/18 16:57, Ming Lei wrote:
>>> Even though RCU lock is held during dispatch, preemption or interrupt
>>> can happen too, so it is simply wrong to depend on the timing to make
>>> sure __blk_mq_run_hw_queue() can see the request in this situation.
>>
>> It is very unlikely that this race will ever be hit because that race exists
>> for less than one microsecond and the smallest timeout that can be specified
>> for delayed queue rerunning is one millisecond. Let's address this race if
>> anyone ever finds a way to hit it.
>
> Please don't depend on the timing which is often fragile, as we can make it
> correct in a generic approach. Also we should avoid to make every driver to
> follow this way for dealing with most of STS_RESOURCE, right?
Wouldn't it be better to fix that race without changing the block layer
API, e.g. by using call_rcu() for delayed queue runs? As you know
call_rcu() will only call the specified function after a grace period.
Since pushing back requests onto the dispatch list happens with the RCU
lock held using call_rcu() for delayed queue runs would be sufficient to
address this race.
Bart.
next prev parent reply other threads:[~2018-01-23 16:37 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-22 3:35 [PATCH 0/5] blk-mq & dm: fix IO hang and deal with one performance issue Ming Lei
2018-01-22 3:35 ` [PATCH 1/5] blk-mq: introduce BLK_STS_DEV_RESOURCE Ming Lei
2018-01-22 16:32 ` Christoph Hellwig
2018-01-22 16:49 ` Bart Van Assche
2018-01-22 16:49 ` Bart Van Assche
2018-01-23 0:57 ` Ming Lei
2018-01-23 16:17 ` Bart Van Assche
2018-01-23 16:26 ` Ming Lei
2018-01-23 16:37 ` Bart Van Assche [this message]
2018-01-23 16:41 ` Ming Lei
2018-01-23 16:47 ` Bart Van Assche
2018-01-23 16:47 ` Bart Van Assche
2018-01-23 16:49 ` Ming Lei
2018-01-23 16:54 ` Bart Van Assche
2018-01-23 16:54 ` Bart Van Assche
2018-01-23 16:59 ` Ming Lei
2018-01-23 16:59 ` Ming Lei
2018-01-23 22:01 ` Bart Van Assche
2018-01-23 22:01 ` Bart Van Assche
2018-01-24 2:31 ` Ming Lei
2018-01-22 3:35 ` [PATCH 2/5] dm-rq: handle dispatch exception in dm_dispatch_clone_request() Ming Lei
2018-01-22 3:35 ` Ming Lei
2018-01-22 3:35 ` [PATCH 3/5] dm-rq: return BLK_STS_* from map_request() Ming Lei
2018-01-22 3:35 ` Ming Lei
2018-01-22 5:35 ` Ming Lei
2018-01-22 3:35 ` [PATCH 4/5] blk-mq: introduce blk_get_request_notify Ming Lei
2018-01-22 10:19 ` Ming Lei
2018-01-22 17:13 ` Bart Van Assche
2018-01-22 17:13 ` Bart Van Assche
2018-01-23 1:29 ` Ming Lei
2018-01-22 3:35 ` [PATCH 5/5] dm-mpath: use blk_mq_alloc_request_notify for allocating blk-mq req Ming Lei
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