block, bfq: reset last_completed_rq_bfqq if the pointed queue is freed

Since commit 13a857a4 ("block, bfq: detect wakers and
unconditionally inject their I/O"), BFQ stores, in a per-device
pointer last_completed_rq_bfqq, the last bfq_queue that had an I/O
request completed. If some bfq_queue receives new I/O right after the
last request of last_completed_rq_bfqq has been completed, then
last_completed_rq_bfqq may be a waker bfq_queue.

But if the bfq_queue last_completed_rq_bfqq points to is freed, then
last_completed_rq_bfqq becomes a dangling reference. This commit
resets last_completed_rq_bfqq if the pointed bfq_queue is freed.

Fixes: 13a857a4 ("block, bfq: detect wakers and unconditionally inject their I/O")
Reported-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -1924,12 +1924,13 @@ static void bfq_add_request(struct request *rq)
 		 * confirmed no later than during the next
 		 * I/O-plugging interval for bfqq.
 		 */
-		if (!bfq_bfqq_has_short_ttime(bfqq) &&
+		if (bfqd->last_completed_rq_bfqq &&
+		    !bfq_bfqq_has_short_ttime(bfqq) &&
 		    ktime_get_ns() - bfqd->last_completion <
 		    200 * NSEC_PER_USEC) {
 			if (bfqd->last_completed_rq_bfqq != bfqq &&
-				   bfqd->last_completed_rq_bfqq !=
-				   bfqq->waker_bfqq) {
+			    bfqd->last_completed_rq_bfqq !=
+			    bfqq->waker_bfqq) {
 				/*
 				 * First synchronization detected with
 				 * a candidate waker queue, or with a
@@ -4808,6 +4809,9 @@ void bfq_put_queue(struct bfq_queue *bfqq)
 			bfqq->bfqd->burst_size--;
 	}
 
+	if (bfqq->bfqd && bfqq->bfqd->last_completed_rq_bfqq == bfqq)
+		bfqq->bfqd->last_completed_rq_bfqq = NULL;
+
 	kmem_cache_free(bfq_pool, bfqq);
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
 	bfqg_and_blkg_put(bfqg);