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

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>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 7 insertions, 1 deletions

diff --git a/block/blk-exec.c b/block/blk-exec.c
index 8b6dc5bd4dd0..f71eac35c1b9 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);
 }