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

Since commit 13a857a4c4e8 ("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: 13a857a4c4e8 ("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>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 586fcfe..b200965 100644 (file)
--- a/block/bfq-iosched.c
+++ b/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);