[PATCH 2/4] block: Fix a race between request queue freezing and running queues

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Any request queue data structure may change while a queue is frozen.
Hence make sure that blk_mq_run_hw_queues() does not access any hw
queue while a request queue is frozen.

After blk_cleanup_queue() has marked a queue as dead it is no longer
safe to access the hardware queue data structures. This patch avoids
that blk_mq_run_hw_queues() crashes when called during or after
blk_cleanup_queue() has freed the hardware queues. This patch is a
variant of a patch posted by Hannes Reinecke ("[PATCH] block: don't
call blk_mq_run_hw_queues() for dead or dying queues "). This patch
is similar in nature to commit c246e80d8673 ("block: Avoid that
request_fn is invoked on a dead queue"; v3.8). An example of a crash
that is fixed by this patch:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8135a10b>] sbitmap_any_bit_set+0xb/0x30
Call Trace:
 [<ffffffff81303a88>] blk_mq_run_hw_queues+0x48/0x90
 [<ffffffff813053cc>] blk_mq_requeue_work+0x10c/0x120
 [<ffffffff81098cb4>] process_one_work+0x154/0x410
 [<ffffffff81099896>] worker_thread+0x116/0x4a0
 [<ffffffff8109edb9>] kthread+0xc9/0xe0
 [<ffffffff81619b05>] ret_from_fork+0x55/0x80

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: James Smart <james.smart@xxxxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: a063057d7c73 ("block: Fix a race between request queue removal and the block cgroup controller") # v4.17.
Reported-by: James Smart <james.smart@xxxxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/blk-mq.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3ff3d7b49969..652d0c6d5945 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1499,12 +1499,20 @@ void blk_mq_run_hw_queues(struct request_queue *q, bool async)
 	struct blk_mq_hw_ctx *hctx;
 	int i;
 
+	/*
+	 * Do not run any hardware queues if the queue is frozen or if a
+	 * concurrent blk_cleanup_queue() call is removing any data
+	 * structures used by this function.
+	 */
+	if (!percpu_ref_tryget(&q->q_usage_counter))
+		return;
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (blk_mq_hctx_stopped(hctx))
 			continue;
 
 		blk_mq_run_hw_queue(hctx, async);
 	}
+	percpu_ref_put(&q->q_usage_counter);
 }
 EXPORT_SYMBOL(blk_mq_run_hw_queues);
 
-- 
2.21.0.196.g041f5ea1cf98




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