blk-mq: fix is_flush_rq

is_flush_rq() is called from bt_iter()/bt_tags_iter(), and runs the
following check:

	hctx->fq->flush_rq == req

but the passed hctx from bt_iter()/bt_tags_iter() may be NULL because:

1) memory re-order in blk_mq_rq_ctx_init():

	rq->mq_hctx = data->hctx;
	...
	refcount_set(&rq->ref, 1);

OR

2) tag re-use and ->rqs[] isn't updated with new request.

Fix the issue by re-writing is_flush_rq() as:

	return rq->end_io == flush_end_io;

which turns out simpler to follow and immune to data race since we have
ordered WRITE rq->end_io and refcount_set(&rq->ref, 1).

Fixes: 2e315dc ("blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter")
Cc: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
Cc: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210818010925.607383-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

block/blk-flush.c

@@ -262,6 +262,11 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
 }
 
+bool is_flush_rq(struct request *rq)
+{
+	return rq->end_io == flush_end_io;
+}
+
 /**
  * blk_kick_flush - consider issuing flush request
  * @q: request_queue being kicked

block/blk-mq.c

@@ -911,7 +911,7 @@ static bool blk_mq_req_expired(struct request *rq, unsigned long *next)
 
 void blk_mq_put_rq_ref(struct request *rq)
 {
-	if (is_flush_rq(rq, rq->mq_hctx))
+	if (is_flush_rq(rq))
 		rq->end_io(rq, 0);
 	else if (refcount_dec_and_test(&rq->ref))
 		__blk_mq_free_request(rq);

block/blk.h

@@ -44,11 +44,7 @@ static inline void __blk_get_queue(struct request_queue *q)
 	kobject_get(&q->kobj);
 }
 
-static inline bool
-is_flush_rq(struct request *req, struct blk_mq_hw_ctx *hctx)
-{
-	return hctx->fq->flush_rq == req;
-}
+bool is_flush_rq(struct request *req);
 
 struct blk_flush_queue *blk_alloc_flush_queue(int node, int cmd_size,
 					      gfp_t flags);