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List:       dm-devel
Subject:    Re: [dm-devel] [PATCH V3] blk-mq: introduce BLK_STS_DEV_RESOURCE
From:       Bart Van Assche <Bart.VanAssche () wdc ! com>
Date:       2018-01-23 16:57:34
Message-ID: 1516726653.3339.20.camel () wdc ! com
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On Wed, 2018-01-24 at 00:37 +0800, Ming Lei wrote:
> On Tue, Jan 23, 2018 at 04:24:20PM +0000, Bart Van Assche wrote:
> > My opinion about this patch is as follows:
> > * Changing a blk_mq_delay_run_hw_queue() call followed by return
> >   BLK_STS_DEV_RESOURCE into return BLK_STS_RESOURCE is wrong because it changes
> >   a guaranteed queue rerun into a queue rerun that may or may not happen
> >   depending on whether or not multiple queue runs happen simultaneously.
> 
> You may not understand the two:
> 
> 1) it is always safe to return BLK_STS_RESOURCE, which will make sure to
> avoid IO hang by blk_mq_delay_run_hw_queue() or blk_mq_run_hw_queue(),
> and using which one depends on SCHED_RESTART.
> 
> 2) if driver can make sure the queue will be rerun after some resource
> is available, either by itself or by blk-mq, it will return BLK_STS_DEV_RESOURCE
> 
> So what is wrong with this way?

Sorry, I swapped BLK_STS_DEV_RESOURCE and BLK_STS_RESOURCE accidentally in my
reply. What I meant is that changing a blk_mq_delay_run_hw_queue() call followed
by return BLK_STS_RESOURCE into BLK_STS_DEV_RESOURCE is wrong and introduces a
race condition in code where there was no race condition.

Bart.

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