BACKPORT: blk-mq: clear stale request in tags->rq[] before freeing one request pool

refcount_inc_not_zero() in bt_tags_iter() still may read one freed
request.

Fix the issue by the following approach:

1) hold a per-tags spinlock when reading ->rqs[tag] and calling
refcount_inc_not_zero in bt_tags_iter()

2) clearing stale request referred via ->rqs[tag] before freeing
request pool, the per-tags spinlock is held for clearing stale
->rq[tag]

So after we cleared stale requests, bt_tags_iter() won't observe
freed request any more, also the clearing will wait for pending
request reference.

The idea of clearing ->rqs[] is borrowed from John Garry's previous
patch and one recent David's patch.

Tested-by: John Garry <john.garry@huawei.com>
Reviewed-by: David Jeffery <djeffery@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>.

Bug: 197804811
Change-Id: If49478d7b05d3f5b0a26966ddf9ae764cf2fb6b0
[Upstream: cherry picked from commit bd63141d585bef14f4caf111f6d0e27fe2300ec6]
[Todd: refactored to avoid breaking KMI ]
Signed-off-by: Pradeep P V K <pragalla@codeaurora.org>
Signed-off-by: Todd Kjos <tkjos@google.com>
Git-commit: bb96e7f45d
Git-repo: https://android.googlesource.com/kernel/common/
Signed-off-by: Pradeep P V K <pragalla@codeaurora.org>

block/blk-mq-tag.c

@@ -215,10 +215,16 @@ struct bt_iter_data {
 static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
 		unsigned int bitnr)
 {
-	struct request *rq = tags->rqs[bitnr];
+	struct request *rq;
+	unsigned long flags;
+	struct ext_blk_mq_tags *etags;
 
+	etags = container_of(tags, struct ext_blk_mq_tags, tags);
+	spin_lock_irqsave(&etags->lock, flags);
+	rq = tags->rqs[bitnr];
 	if (!rq || !refcount_inc_not_zero(&rq->ref))
-		return NULL;
+		rq = NULL;
+	spin_unlock_irqrestore(&etags->lock, flags);
 	return rq;
 }
 
@@ -486,18 +492,21 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 				     int node, int alloc_policy)
 {
 	struct blk_mq_tags *tags;
+	struct ext_blk_mq_tags *etags;
 
 	if (total_tags > BLK_MQ_TAG_MAX) {
 		pr_err("blk-mq: tag depth too large\n");
 		return NULL;
 	}
 
-	tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
-	if (!tags)
+	etags = kzalloc_node(sizeof(*etags), GFP_KERNEL, node);
+	if (!etags)
 		return NULL;
 
+	tags = &etags->tags;
 	tags->nr_tags = total_tags;
 	tags->nr_reserved_tags = reserved_tags;
+	spin_lock_init(&etags->lock);
 
 	return blk_mq_init_bitmap_tags(tags, node, alloc_policy);
 }

block/blk-mq-tag.h

@@ -21,6 +21,21 @@ struct blk_mq_tags {
 	struct list_head page_list;
 };
 
+/*
+ * Extended tag address space map. This was needed
+ * to add a spinlock to blk_mq_tags in a KMI compliant
+ * way (no changes could be made to struct blk_mq_tags).
+ */
+struct ext_blk_mq_tags {
+	struct blk_mq_tags tags;
+
+	 /*
+	  * used to clear request reference in rqs[] before freeing one
+	  * request pool
+	  */
+	spinlock_t lock;
+};
+
 
 extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags, unsigned int reserved_tags, int node, int alloc_policy);
 extern void blk_mq_free_tags(struct blk_mq_tags *tags);

block/blk-mq.c

@@ -1975,6 +1975,47 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 	}
 }
 
+static size_t order_to_size(unsigned int order)
+{
+	return (size_t)PAGE_SIZE << order;
+}
+
+/* called before freeing request pool in @tags */
+static void blk_mq_clear_rq_mapping(struct blk_mq_tag_set *set,
+		struct blk_mq_tags *tags, unsigned int hctx_idx)
+{
+	struct blk_mq_tags *drv_tags = set->tags[hctx_idx];
+	struct ext_blk_mq_tags *drv_etags;
+	struct page *page;
+	unsigned long flags;
+
+	list_for_each_entry(page, &tags->page_list, lru) {
+		unsigned long start = (unsigned long)page_address(page);
+		unsigned long end = start + order_to_size(page->private);
+		int i;
+
+		for (i = 0; i < set->queue_depth; i++) {
+			struct request *rq = drv_tags->rqs[i];
+			unsigned long rq_addr = (unsigned long)rq;
+
+			if (rq_addr >= start && rq_addr < end) {
+				WARN_ON_ONCE(refcount_read(&rq->ref) != 0);
+				cmpxchg(&drv_tags->rqs[i], rq, NULL);
+			}
+		}
+	}
+
+	/*
+	 * Wait until all pending iteration is done.
+	 *
+	 * Request reference is cleared and it is guaranteed to be observed
+	 * after the ->lock is released.
+	 */
+	drv_etags = container_of(drv_tags, struct ext_blk_mq_tags, tags);
+	spin_lock_irqsave(&drv_etags->lock, flags);
+	spin_unlock_irqrestore(&drv_etags->lock, flags);
+}
+
 static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
 	const int is_sync = op_is_sync(bio->bi_opf);
@@ -2100,6 +2141,8 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		}
 	}
 
+	blk_mq_clear_rq_mapping(set, tags, hctx_idx);
+
 	while (!list_empty(&tags->page_list)) {
 		page = list_first_entry(&tags->page_list, struct page, lru);
 		list_del_init(&page->lru);
@@ -2159,11 +2202,6 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
 	return tags;
 }
 
-static size_t order_to_size(unsigned int order)
-{
-	return (size_t)PAGE_SIZE << order;
-}
-
 static int blk_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
 			       unsigned int hctx_idx, int node)
 {