[PATCHv2 3/3] block: protect debugfs attribute method hctx_busy_show

[Nilay Shroff <nilay@linux.ibm.com>]

The hctx_busy_show method in debugfs is currently unprotected.
This method iterates over all started requests in a tagset and
prints them. However, the tags can be updated concurrently via
the sysfs attributes 'nr_requests' or 'scheduler' (elevator
switch), leading to potential race conditions.

Since sysfs attributes 'nr_requests' and 'scheduler' are already
protected using q->elevator_lock, extend this protection to the
debugfs 'busy' attribute as well to ensure consistency.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-mq-debugfs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 1c958bbaddce..3421b5521fe2 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -347,9 +347,14 @@ static int hctx_busy_show(void *data, struct seq_file *m)
 {
 	struct blk_mq_hw_ctx *hctx = data;
 	struct show_busy_params params = { .m = m, .hctx = hctx };
+	int res;
 
+	res = mutex_lock_interruptible(&hctx->queue->elevator_lock);
+	if (res)
+		return res;
 	blk_mq_tagset_busy_iter(hctx->queue->tag_set, hctx_show_busy_rq,
 				&params);
+	mutex_unlock(&hctx->queue->elevator_lock);
 
 	return 0;
 }
-- 
2.47.1