[PATCH 06/11] block: fix queue freeze vs limits lock order in sysfs store methods

[Christoph Hellwig <hch@lst.de>]

queue_attr_store() always freezes a device queue before calling the
attribute store operation. For attributes that control queue limits, the
store operation will also lock the queue limits with a call to
queue_limits_start_update(). However, some drivers (e.g. SCSI sd) may
need to issue commands to a device to obtain limit values from the
hardware with the queue limits locked. This creates a potential ABBA
deadlock situation if a user attempts to modify a limit (thus freezing
the device queue) while the device driver starts a revalidation of the
device queue limits.

Avoid such deadlock by not freezing the queue before calling the
->store_limit() method in struct queue_sysfs_entry and instead use the
queue_limits_commit_update_frozen helper to freeze the queue after taking
the limits lock.

This also removes taking the sysfs lock for the store_limit method as
it doesn't protect anything here, but creates even more nesting.
Hopefully it will go away from the actual sysfs methods entirely soon.

(commit log adapted from a similar patch from  Damien Le Moal)

Fixes: ff956a3be95b ("block: use queue_limits_commit_update in queue_discard_max_store")
Fixes: 0327ca9d53bf ("block: use queue_limits_commit_update in queue_max_sectors_store")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 block/blk-sysfs.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index d2aa2177e4ba..e828be777206 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -694,22 +694,24 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
 	if (entry->load_module)
 		entry->load_module(disk, page, length);
 
-	mutex_lock(&q->sysfs_lock);
-	blk_mq_freeze_queue(q);
 	if (entry->store_limit) {
 		struct queue_limits lim = queue_limits_start_update(q);
 
 		res = entry->store_limit(disk, page, length, &lim);
 		if (res < 0) {
 			queue_limits_cancel_update(q);
-		} else {
-			res = queue_limits_commit_update(q, &lim);
-			if (!res)
-				res = length;
+			return res;
 		}
-	} else {
-		res = entry->store(disk, page, length);
+
+		res = queue_limits_commit_update_frozen(q, &lim);
+		if (res)
+			return res;
+		return length;
 	}
+
+	mutex_lock(&q->sysfs_lock);
+	blk_mq_freeze_queue(q);
+	res = entry->store(disk, page, length);
 	blk_mq_unfreeze_queue(q);
 	mutex_unlock(&q->sysfs_lock);
 	return res;
-- 
2.45.2