Re: [PATCH V10 11/11] block: deactivate hctx when the hctx is actually inactive

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

On Mon, May 11, 2020 at 01:52:14PM -0700, Bart Van Assche wrote:
> On 2020-05-10 21:08, Ming Lei wrote:
> > OK, just forgot the whole story, but the issue can be fixed quite easily
> > by adding a new request allocation flag in slow path, see the following
> > patch:
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index ec50d7e6be21..d743be1b45a2 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -418,6 +418,11 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
> >  		if (success)
> >  			return 0;
> >  
> > +		if (flags & BLK_MQ_REQ_FORCE) {
> > +			percpu_ref_get(ref);
> > +			return 0;
> > +		}
> > +
> >  		if (flags & BLK_MQ_REQ_NOWAIT)
> >  			return -EBUSY;
> >  
> > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> > index c2ea0a6e5b56..2816886d0bea 100644
> > --- a/include/linux/blk-mq.h
> > +++ b/include/linux/blk-mq.h
> > @@ -448,6 +448,13 @@ enum {
> >  	BLK_MQ_REQ_INTERNAL	= (__force blk_mq_req_flags_t)(1 << 2),
> >  	/* set RQF_PREEMPT */
> >  	BLK_MQ_REQ_PREEMPT	= (__force blk_mq_req_flags_t)(1 << 3),
> > +
> > +	/*
> > +	 * force to allocate request and caller has to make sure queue
> > +	 * won't be forzen completely during allocation, and this flag
> > +	 * is only applied after queue freeze is started
> > +	 */
> > +	BLK_MQ_REQ_FORCE	= (__force blk_mq_req_flags_t)(1 << 4),
> >  };
> >  
> >  struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
> 
> I'm not sure that introducing such a flag is a good idea. After
> blk_mq_freeze_queue() has made it clear that a request queue must be
> frozen and before the request queue is really frozen, an RCU grace
> period must expire. Otherwise it cannot be guaranteed that the intention
> to freeze a request queue (by calling percpu_ref_kill()) has been
> observed by all potential blk_queue_enter() callers (blk_queue_enter()
> calls percpu_ref_tryget_live()). Not introducing any new race conditions
> would either require to introduce an smp_mb() call in blk_queue_enter()
> or to let another RCU grace period expire after the last allocation of a
> request with BLK_MQ_REQ_FORCE and before the request queue is really frozen.

Actually neither smp_mb() or extra grace period is needed, and it can
be explained in the following way simply:

percpu_ref_get() -> percpu_ref_get_many() is introduced by BLK_MQ_REQ_FORCE.

When percpu_ref_get() is called:

- if it is still in percpu mode, it will be covered by the rcu grace period
in percpu_ref_kill_and_confirm().

- otherwise, the refcount is grabbed in atomic mode, no extra smp_mb()
or rcu period required because we guarantee that the atomic number is >
1 when calling percpu_ref_get(). And blk_mq_freeze_queue_wait() will
observe correct value of this atomic refcount.

percpu_ref_get() is documented as :

 * This function is safe to call as long as @ref is between init and exit.


Thanks,
Ming