blk-mq: allow non-softirq completions

Right now we export two ways of completing a request:

1) blk_mq_complete_request(). This uses an IPI (if needed) and
   completes through q->softirq_done_fn(). It also works with
   timeouts.

2) blk_mq_end_io(). This completes inline, and ignores any timeout
   state of the request.

Let blk_mq_complete_request() handle non-softirq_done_fn completions
as well, by just completing inline. If a driver has enough completion
ports to place completions correctly, it need not define a
mq_ops->complete() and we can avoid an indirect function call by
doing the completion inline.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -434,10 +434,16 @@ void __blk_mq_complete_request(struct request *rq)
  **/
 void blk_mq_complete_request(struct request *rq)
 {
-	if (unlikely(blk_should_fake_timeout(rq->q)))
+	struct request_queue *q = rq->q;
+
+	if (unlikely(blk_should_fake_timeout(q)))
 		return;
-	if (!blk_mark_rq_complete(rq))
+	if (!blk_mark_rq_complete(rq)) {
+		if (q->softirq_done_fn)
 			__blk_mq_complete_request(rq);
+		else
+			blk_mq_end_io(rq, rq->errors);
+	}
 }
 EXPORT_SYMBOL(blk_mq_complete_request);
 

include/linux/blk-mq.h

@@ -173,6 +173,10 @@ void __blk_mq_end_io(struct request *rq, int error);
 
 void blk_mq_requeue_request(struct request *rq);
 
+/*
+ * Complete request through potential IPI for right placement. Driver must
+ * have defined a mq_ops->complete() hook for this.
+ */
 void blk_mq_complete_request(struct request *rq);
 
 void blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx);