linux-kernel - [ 80/89] block: Dont access request after it might be freed

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Date:	Mon, 03 Dec 2012 14:33:06 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Roland Dreier <roland@...estorage.com>,
	Jens Axboe <axboe@...nel.dk>
Subject: [ 80/89] block: Dont access request after it might be freed

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Roland Dreier <roland@...estorage.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@...estorage.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 block/blk-exec.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/block/blk-exec.c
+++ b/block/blk-exec.c
@@ -49,6 +49,7 @@ void blk_execute_rq_nowait(struct reques
 			   rq_end_io_fn *done)
 {
 	int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
+	bool is_pm_resume;
 
 	if (unlikely(blk_queue_dead(q))) {
 		rq->errors = -ENXIO;
@@ -59,12 +60,18 @@ void blk_execute_rq_nowait(struct reques
 
 	rq->rq_disk = bd_disk;
 	rq->end_io = done;
+	/*
+	 * 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;
+
 	WARN_ON(irqs_disabled());
 	spin_lock_irq(q->queue_lock);
 	__elv_add_request(q, rq, where);
 	__blk_run_queue(q);
 	/* the queue is stopped so it won't be run */
-	if (rq->cmd_type == REQ_TYPE_PM_RESUME)
+	if (is_pm_resume)
 		q->request_fn(q);
 	spin_unlock_irq(q->queue_lock);
 }


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