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

Roland Dreier
2012-11-22T19:20:16Z

From: Roland Dreier <roland@purestorage.com>

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@purestorage.com>
---
block/blk-exec.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/blk-exec.c b/block/blk-exec.c
index 8b6dc5b..f71eac3 100644
--- a/block/blk-exec.c
+++ b/block/blk-exec.c
@@ -52,11 +52,17 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
rq_end_io_fn *done)
{
int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
+ bool is_pm_resume;

WARN_ON(irqs_disabled());

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;

spin_lock_irq(q->queue_lock);

@@ -71,7 +77,7 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
__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);
}
--
1.7.10.4