Blk-iolatency: warn on negative inflight IO counter

This is to catch any unexpected negative value of inflight IO counter.
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iolatency.c

@@ -592,6 +592,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
 	u64 now = ktime_to_ns(ktime_get());
 	bool issue_as_root = bio_issue_as_root_blkg(bio);
 	bool enabled = false;
+	int inflight = 0;
 
 	blkg = bio->bi_blkg;
 	if (!blkg || !bio_flagged(bio, BIO_TRACKED))
@@ -613,7 +614,8 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
 		}
 		rqw = &iolat->rq_wait;
 
-		atomic_dec(&rqw->inflight);
+		inflight = atomic_dec_return(&rqw->inflight);
+		WARN_ON_ONCE(inflight < 0);
 		if (iolat->min_lat_nsec == 0)
 			goto next;
 		iolatency_record_time(iolat, &bio->bi_issue, now,