[RFC PATCHv2 2/2] block: fix nr_hw_queue update racing with disk addition/removal

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

The nr_hw_queue update could potentially race with disk addtion/removal
while registering/unregistering hctx sysfs files. The __blk_mq_update_
nr_hw_queues() runs with q->tag_list_lock held and so to avoid it racing
with disk addition/removal we should acquire q->tag_list_lock while
registering/unregistering hctx sysfs files.

With this patch, blk_mq_sysfs_register() (called during disk addition)
and blk_mq_sysfs_unregister() (called during disk removal) now runs
with q->tag_list_lock held so that it avoids racing with __blk_mq_update
_nr_hw_queues().

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-mq-sysfs.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 6113328abd70..2b9df2b73bf6 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -229,22 +229,31 @@ int blk_mq_sysfs_register(struct gendisk *disk)
 
 	kobject_uevent(q->mq_kobj, KOBJ_ADD);
 
+	mutex_lock(&q->tag_set->tag_list_lock);
+
 	queue_for_each_hw_ctx(q, hctx, i) {
 		ret = blk_mq_register_hctx(hctx);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&q->tag_set->tag_list_lock);
 			goto unreg;
+		}
 	}
 
+	mutex_unlock(&q->tag_set->tag_list_lock);
 
 out:
 	return ret;
 
 unreg:
+	mutex_lock(&q->tag_set->tag_list_lock);
+
 	queue_for_each_hw_ctx(q, hctx, j) {
 		if (j < i)
 			blk_mq_unregister_hctx(hctx);
 	}
 
+	mutex_unlock(&q->tag_set->tag_list_lock);
+
 	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
 	kobject_del(q->mq_kobj);
 	return ret;
@@ -256,10 +265,13 @@ void blk_mq_sysfs_unregister(struct gendisk *disk)
 	struct blk_mq_hw_ctx *hctx;
 	unsigned long i;
 
+	mutex_lock(&q->tag_set->tag_list_lock);
 
 	queue_for_each_hw_ctx(q, hctx, i)
 		blk_mq_unregister_hctx(hctx);
 
+	mutex_unlock(&q->tag_set->tag_list_lock);
+
 	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
 	kobject_del(q->mq_kobj);
 }
-- 
2.47.1