[PATCH] block: protect debugfs attributes using q->elevator_lock

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

Currently, the block debugfs attributes (tags, tags_bitmap, sched_tags, 
and sched_tags_bitmap) are protected using q->sysfs_lock. However, these 
attributes are updated in multiple scenarios:
- During driver probe method
- During an elevator switch/update
- During an nr_hw_queues update
- When writing to the sysfs attribute nr_requests

All these update paths (except driver probe method which anyways doesn't
not require any protection) are already protected using q->elevator_lock
.So to ensure consistency and proper synchronization, replace q->sysfs_
lock with q->elevator_lock for protecting these debugfs attributes.

Additionally, debugfs attribute "busy" is currently unprotected. This
attribute iterates over all started requests in a tagset and prints them. 
However, the tags can be updated simultaneously via the sysfs attribute 
"nr_requests" or "scheduler" (elevator switch), leading to potential race 
conditions. Since the sysfs attributes "nr_requests" and "scheduler" are 
already protected using q->elevator_lock, extend this protection to the 
debugfs "busy" attribute as well.

This change ensures that all relevant debugfs attributes are properly
synchronized, preventing potential data inconsistencies.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
Please note that this patch was unit tested against blktests and 
quick xfstests.

---
 block/blk-mq-debugfs.c | 25 +++++++++++++++----------
 include/linux/blkdev.h |  6 +++---
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index adf5f0697b6b..d26a6df945bd 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -347,11 +347,16 @@ 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)
+		goto out;
 	blk_mq_tagset_busy_iter(hctx->queue->tag_set, hctx_show_busy_rq,
 				&params);
-
-	return 0;
+	mutex_unlock(&hctx->queue->elevator_lock);
+out:
+	return res;
 }
 
 static const char *const hctx_types[] = {
@@ -400,12 +405,12 @@ static int hctx_tags_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->tags)
 		blk_mq_debugfs_tags_show(m, hctx->tags);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -417,12 +422,12 @@ static int hctx_tags_bitmap_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->tags)
 		sbitmap_bitmap_show(&hctx->tags->bitmap_tags.sb, m);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -434,12 +439,12 @@ static int hctx_sched_tags_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->sched_tags)
 		blk_mq_debugfs_tags_show(m, hctx->sched_tags);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -451,12 +456,12 @@ static int hctx_sched_tags_bitmap_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->sched_tags)
 		sbitmap_bitmap_show(&hctx->sched_tags->bitmap_tags.sb, m);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 22600420799c..709a32022c78 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -568,9 +568,9 @@ struct request_queue {
 	 * nr_requests and wbt latency, this lock also protects the sysfs attrs
 	 * nr_requests and wbt_lat_usec. Additionally the nr_hw_queues update
 	 * may modify hctx tags, reserved-tags and cpumask, so this lock also
-	 * helps protect the hctx attrs. To ensure proper locking order during
-	 * an elevator or nr_hw_queue update, first freeze the queue, then
-	 * acquire ->elevator_lock.
+	 * helps protect the hctx sysfs/debugfs attrs. To ensure proper locking
+	 * order during an elevator or nr_hw_queue update, first freeze the
+	 * queue, then acquire ->elevator_lock.
 	 */
 	struct mutex		elevator_lock;
 
-- 
2.47.1