It has been reported many times that a use-after-free can be intermittently
found when iterating busy requests:

- https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@huawei.com/ (local)
- https://lore.kernel.org/linux-block/5c3ac5af-ed81-11e4-fee3-f92175f14daf@acm.org/T/#m6c1ac11540522716f645d004e2a5a13c9f218908 (local)
- https://lore.kernel.org/linux-block/04e2f9e8-79fa-f1cb-ab23-4a15bf3f64cc@kernel.dk/ (local)

The issue is that when we switch scheduler or change queue nr_requests,
the driver tagset may keep references to the stale requests.

As a solution, clean up any references to those requests in the driver
tagset when freeing. This is done with a cmpxchg to make safe any race
with setting the driver tagset request from another queue.

Signed-off-by: John Garry <redacted>
--
Set as RFC as I need to test more. And not sure on solution method, as
Bart had another idea.

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index d1eafe2c045c..9b042c7036b3 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c

@@ -621,7 +621,7 @@ void blk_mq_sched_free_requests(struct request_queue *q)
 
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (hctx->sched_tags)
-			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
+			blk_mq_free_rqs_ext(q->tag_set, hctx->sched_tags, i, hctx->tags);
 	}
 }
 

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 9c92053e704d..562db72e7d79 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c

@@ -576,7 +576,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 			return -ENOMEM;
 		}
 
-		blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
+		blk_mq_free_rqs_ext(set, *tagsptr, hctx->queue_num, hctx->tags);
 		blk_mq_free_rq_map(*tagsptr, flags);
 		*tagsptr = new;
 	} else {

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 55bcee5dc032..f3aad695cd25 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2271,8 +2271,8 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 	return BLK_QC_T_NONE;
 }
 
-void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
-		     unsigned int hctx_idx)
+void blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
+		     unsigned int hctx_idx, struct blk_mq_tags *references)
 {
 	struct page *page;
 

@@ -2281,10 +2281,13 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 
 		for (i = 0; i < tags->nr_tags; i++) {
 			struct request *rq = tags->static_rqs[i];
+			int j;
 
 			if (!rq)
 				continue;
 			set->ops->exit_request(set, rq, hctx_idx);
+			for (j = 0; references && j < references->nr_tags; j++)
+				cmpxchg(&references->rqs[j], rq, 0);