[PATCH] block: fix possible race on blk_get_queue()

[Luis Chamberlain <mcgrof@kernel.org>]

The queue can flip to dying after we check if it is dying,
and then we call __blk_get_queue(). This is a purely
theoretical race, but just fix it. We do this by
Using the atomic kobject_get_unless_zero() first, and
*then* check if the queue is dying *after*.

This issue was found while doing patch review on the
recent blktrace fixes [0].

[0] https://lore.kernel.org/linux-block/20200415123434.GU11244@42.do-not-panic.com/

Reported-by: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---

This goes tested against blktest without finding a regression.

 block/blk-core.c | 14 ++++++++++----
 block/blk.h      |  5 +++--
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index d9d632639bd1..febdd8e8d409 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -605,12 +605,18 @@ EXPORT_SYMBOL(blk_alloc_queue);
  */
 bool blk_get_queue(struct request_queue *q)
 {
-	if (likely(!blk_queue_dying(q))) {
-		__blk_get_queue(q);
-		return true;
+	struct kobject *obj;
+
+	obj = __blk_get_queue(q);
+	if (!obj)
+		return false;
+
+	if (unlikely(blk_queue_dying(q))) {
+		blk_put_queue(q);
+		return false;
 	}
 
-	return false;
+	return true;
 }
 EXPORT_SYMBOL(blk_get_queue);
 
diff --git a/block/blk.h b/block/blk.h
index 49e2928a1632..bdbc9b084d5b 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -39,9 +39,10 @@ blk_get_flush_queue(struct request_queue *q, struct blk_mq_ctx *ctx)
 	return blk_mq_map_queue(q, REQ_OP_FLUSH, ctx)->fq;
 }
 
-static inline void __blk_get_queue(struct request_queue *q)
+static inline struct kobject * __must_check
+__blk_get_queue(struct request_queue *q)
 {
-	kobject_get(&q->kobj);
+	return kobject_get_unless_zero(&q->kobj);
 }
 
 static inline bool
-- 
2.27.0