[PATCH 139/241] block: Don't access request after it might be freed

Herton Ronaldo Krzesinski herton.krzesinski at canonical.com
Thu Dec 13 13:58:24 UTC 2012 -stable review patch.  If anyone has any objections, please let me know.


From: Roland Dreier <roland at purestorage.com>

commit 893d290f1d7496db97c9471bc352ad4a11dc8a25 upstream.

After we've done __elv_add_request() and __blk_run_queue() in
blk_execute_rq_nowait(), the request might finish and be freed
immediately.  Therefore checking if the type is REQ_TYPE_PM_RESUME
isn't safe afterwards, because if it isn't, rq might be gone.
Instead, check beforehand and stash the result in a temporary.

This fixes crashes in blk_execute_rq_nowait() I get occasionally when
running with lots of memory debugging options enabled -- I think this
race is usually harmless because the window for rq to be reallocated
is so small.

Signed-off-by: Roland Dreier <roland at purestorage.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
[ herton: adjust context ]
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
 block/blk-exec.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/blk-exec.c b/block/blk-exec.c
index fb2cbd5..9925fbe 100644
--- a/block/blk-exec.c
+++ b/block/blk-exec.c
@@ -49,8 +49,16 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
 			   rq_end_io_fn *done)
+	bool is_pm_resume;
+	/*
+	 * need to check this before __blk_run_queue(), because rq can
+	 * be freed before that returns.
+	 */
+	is_pm_resume = rq->cmd_type == REQ_TYPE_PM_RESUME;
 	if (unlikely(blk_queue_dead(q))) {
@@ -66,7 +74,7 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
 	__elv_add_request(q, rq, where);
 	/* the queue is stopped so it won't be run */
-	if (rq->cmd_type == REQ_TYPE_PM_RESUME)
+	if (is_pm_resume)

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