[PATCH V4 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

[Sagi Grimberg]

>>   void blk_mq_quiesce_queue(struct request_queue *q)
>>   {
>> -	struct blk_mq_hw_ctx *hctx;
>> -	unsigned int i;
>> -	bool rcu = false;
>> +	bool blocking = !!(q->tag_set->flags & BLK_MQ_F_BLOCKING);
>> +	bool was_quiesced =__blk_mq_quiesce_queue_nowait(q);
>>   
>> -	__blk_mq_quiesce_queue_nowait(q);
>> +	if (!was_quiesced && blocking)
>> +		percpu_ref_kill(&q->dispatch_counter);
>>   
>> -	queue_for_each_hw_ctx(q, hctx, i) {
>> -		if (hctx->flags & BLK_MQ_F_BLOCKING)
>> -			synchronize_srcu(hctx->srcu);
>> -		else
>> -			rcu = true;
>> -	}
>> -	if (rcu)
>> +	if (blocking)
>> +		wait_event(q->mq_quiesce_wq,
>> +				percpu_ref_is_zero(&q->dispatch_counter));
>> +	else
>>   		synchronize_rcu();
>>   }
> 
> In the previous version, you had ensured no thread can unquiesce a queue
> while another is waiting for quiescence. Now that the locking is gone,
> a thread could unquiesce the queue before percpu_ref reaches zero, so
> the wait_event() may never complete on the resurrected percpu_ref.

Yea, where did that go?