[PATCH 3/5] blk-mq: add helper of blk_mq_global_quiesce_wait()

[Sagi Grimberg]

>>>>> Add helper of blk_mq_global_quiesce_wait() for supporting to quiesce
>>>>> queues in parallel, then we can just wait once if global quiesce wait
>>>>> is allowed.
>>>>
>>>> blk_mq_global_quiesce_wait() is a poor name... global is scope-less and
>>>> obviously it has a scope.
>>>
>>> How about blk_mq_shared_quiesce_wait()? or any suggestion?
>>
>> Shared between what?
> 
> All request queues in one host-wide, both scsi and nvme has such
> requirement.
> 
>>
>> Maybe if the queue has a non-blocking tagset, it can have a "quiesced"
>> flag that is cleared in unquiesce? then the callers can just continue
>> to iterate but will only wait the rcu grace period once.
> 
> Yeah, that is what these patches try to implement.

I was suggesting to "hide" it in the interface.
Maybe something like:
--
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8799fa73ef34..627b631db1f9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -263,14 +263,18 @@ void blk_mq_wait_quiesce_done(struct request_queue *q)
         unsigned int i;
         bool rcu = false;

+       if (!q->has_srcu && q->quiesced)
+               return;
         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 (rcu) {
                 synchronize_rcu();
+               q->quiesced = true;
+       }
  }
  EXPORT_SYMBOL_GPL(blk_mq_wait_quiesce_done);

@@ -308,6 +312,7 @@ void blk_mq_unquiesce_queue(struct request_queue *q)
         } else if (!--q->quiesce_depth) {
                 blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
                 run_queue = true;
+               q->quiesced = false;
         }
         spin_unlock_irqrestore(&q->queue_lock, flags);
--