[dm-devel] [PATCH V3] blk-mq: introduce BLK_STS_DEV_RESOURCE

[Ming Lei]

On Tue, Jan 23, 2018 at 04:57:34PM +0000, Bart Van Assche wrote:
> 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.

OK, then no such race you worried about in this patch.

Jens, could you take a look at this patch?

Thanks,
Ming