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