block, bfq: reset waker pointer with shared queues

Commit 85686d0dc1 ("block, bfq: keep shared queues out of the waker
mechanism") leaves shared bfq_queues out of the waker-detection
mechanism. It attains this goal by not updating the pointer
last_completed_rq_bfqq, if the last request completed belongs to a
shared bfq_queue (so that the pointer will not point to the shared
bfq_queue).

Yet this has a side effect: the pointer last_completed_rq_bfqq keeps
pointing, deceptively, to a bfq_queue that actually is not the last
one to have had a request completed. As a consequence, such a
bfq_queue may deceptively be considered as a waker of some bfq_queue,
even of some shared bfq_queue.

To address this issue, reset last_completed_rq_bfqq if the last
request completed belongs to a shared queue.

Fixes: 85686d0dc1 ("block, bfq: keep shared queues out of the waker mechanism")
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-8-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -6165,11 +6165,13 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
 	 * of other queues. But a false waker will unjustly steal
 	 * bandwidth to its supposedly woken queue. So considering
 	 * also shared queues in the waking mechanism may cause more
-	 * control troubles than throughput benefits. Then do not set
-	 * last_completed_rq_bfqq to bfqq if bfqq is a shared queue.
+	 * control troubles than throughput benefits. Then reset
+	 * last_completed_rq_bfqq if bfqq is a shared queue.
 	 */
 	if (!bfq_bfqq_coop(bfqq))
 		bfqd->last_completed_rq_bfqq = bfqq;
+	else
+		bfqd->last_completed_rq_bfqq = NULL;
 
 	/*
 	 * If we are waiting to discover whether the request pattern