Re: [PATCH 1/5] blk-mq: introduce BLK_STS_DEV_RESOURCE

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jan 23, 2018 at 08:37:26AM -0800, Bart Van Assche wrote:
> 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

Could you explain where to call call_rcu()?  call_rcu() can't be used in
IO path at all.

> 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.


-- 
Ming