[PATCH 2/2] block: hold queue lock while iterating in diskstats_show

[Daniel Wagner <dwagner@suse.de>]

The request queues can be freed while we operate on them. Make sure we
hold a reference when using blk_mq_queue_tag_busy_iter.

RIP: blk_mq_queue_tag_busy_iter
Call Trace:
 ? blk_mq_hctx_mark_pending
 ? diskstats_show
 ? blk_mq_hctx_mark_pending
 blk_mq_in_flight
 diskstats_show
 ? klist_next
 seq_read
 proc_reg_read
 vfs_read
 ksys_read
 do_syscall_64
 entry_SYSCALL_64_after_hwframe

Signed-off-by: Daniel Wagner <dwagner@suse.de>
---
 block/genhd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/genhd.c b/block/genhd.c
index c9d4386dd177..0e163055a4e6 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1167,10 +1167,13 @@ static int diskstats_show(struct seq_file *seqf, void *v)
 		if (bdev_is_partition(hd) && !bdev_nr_sectors(hd))
 			continue;
 		part_stat_read_all(hd, &stat);
+		if (blk_queue_enter(gp->queue, BLK_MQ_REQ_NOWAIT))
+			continue;
 		if (queue_is_mq(gp->queue))
 			inflight = blk_mq_in_flight(gp->queue, hd);
 		else
 			inflight = part_in_flight(hd);
+		blk_queue_exit(gp->queue);
 
 		seq_printf(seqf, "%4d %7d %pg "
 			   "%lu %lu %lu %u "
-- 
2.29.2