[PATCH][SRU][Artful] blk-mq-tag: check for NULL rq when iterating tags

[Guilherme G. Piccoli]

From: Jens Axboe <axboe at kernel.dk>

BugLink: https://bugs.launchpad.net/bugs/1744300

Since we introduced blk-mq-sched, the tags->rqs[] array has been
dynamically assigned. So we need to check for NULL when iterating,
since there's a window of time where the bit is set, but we haven't
dynamically assigned the tags->rqs[] array position yet.

This is perfectly safe, since the memory backing of the request is
never going away while the device is alive.

Reviewed-by: Bart Van Assche <bart.vanassche at wdc.com>
Reviewed-by: Omar Sandoval <osandov at fb.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry-pick from commit 7f5562d5ecc44c757599b201df928ba52fa05047)
Signed-off-by: Guilherme G. Piccoli <gpiccoli at canonical.com>
---
 block/blk-mq-tag.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d0be72ccb091..dc9e6dac5a2a 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -214,7 +214,11 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 		bitnr += tags->nr_reserved_tags;
 	rq = tags->rqs[bitnr];
 
-	if (rq->q == hctx->queue)
+	/*
+	 * We can hit rq == NULL here, because the tagging functions
+	 * test and set the bit before assining ->rqs[].
+	 */
+	if (rq && rq->q == hctx->queue)
 		iter_data->fn(hctx, rq, iter_data->data, reserved);
 	return true;
 }
@@ -248,9 +252,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 
 	if (!reserved)
 		bitnr += tags->nr_reserved_tags;
+
+	/*
+	 * We can hit rq == NULL here, because the tagging functions
+	 * test and set the bit before assining ->rqs[].
+	 */
 	rq = tags->rqs[bitnr];
+	if (rq)
+		iter_data->fn(rq, iter_data->data, reserved);
 
-	iter_data->fn(rq, iter_data->data, reserved);
 	return true;
 }
 
-- 
2.15.1