block: protect debugfs attribute method hctx_busy_show

JIRA: https://issues.redhat.com/browse/RHEL-112997

commit 0e94ed3
Author: Nilay Shroff <nilay@linux.ibm.com>
Date:   Thu Mar 13 17:21:52 2025 +0530

    block: protect debugfs attribute method hctx_busy_show

    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>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/r/20250313115235.3707600-4-nilay@linux.ibm.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

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;
 }