[PATCH 3/4] blk-rq-qos: fix possible deadlock

[Yu Kuai]

Currently rq-qos debugfs entries is created from rq_qos_add(), while
rq_qos_add() requires queue to be freezed. This can deadlock because
creating new entries can trigger fs reclaim.

Fix this problem by delaying creating rq-qos debugfs entries until
it's initialization is complete.

- For wbt, it can be initialized by default of by blk-sysfs, fix it by
  calling blk_mq_debugfs_register_rq_qos() after wbt_init;
- For other policies, they can only be initialized by blkg configuration,
  fix it by calling blk_mq_debugfs_register_rq_qos() from
  blkg_conf_end();

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 block/blk-cgroup.c | 6 ++++++
 block/blk-rq-qos.c | 7 -------
 block/blk-sysfs.c  | 4 ++++
 block/blk-wbt.c    | 7 ++++++-
 4 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d93654334854..e4ccabf132c0 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,6 +33,7 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
+#include "blk-mq-debugfs.h"
 
 static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu);
 
@@ -746,6 +747,11 @@ void blkg_conf_end(struct blkg_conf_ctx *ctx)
 	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, ctx->memflags);
 	blkdev_put_no_open(ctx->bdev);
+
+	mutex_lock(&q->debugfs_mutex);
+	blk_mq_debugfs_register_rq_qos(q);
+	mutex_unlock(&q->debugfs_mutex);
+
 }
 EXPORT_SYMBOL_GPL(blkg_conf_end);
 
diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 654478dfbc20..d7ce99ce2e80 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -347,13 +347,6 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
 	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
 
 	blk_mq_unfreeze_queue(q, memflags);
-
-	if (rqos->ops->debugfs_attrs) {
-		mutex_lock(&q->debugfs_mutex);
-		blk_mq_debugfs_register_rqos(rqos);
-		mutex_unlock(&q->debugfs_mutex);
-	}
-
 	return 0;
 ebusy:
 	blk_mq_unfreeze_queue(q, memflags);
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 76c47fe9b8d6..52bb4db25cf5 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -688,6 +688,10 @@ static ssize_t queue_wb_lat_store(struct gendisk *disk, const char *page,
 	mutex_unlock(&disk->rqos_state_mutex);
 
 	blk_mq_unquiesce_queue(q);
+
+	mutex_lock(&q->debugfs_mutex);
+	blk_mq_debugfs_register_rq_qos(q);
+	mutex_unlock(&q->debugfs_mutex);
 out:
 	blk_mq_unfreeze_queue(q, memflags);
 
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index eb8037bae0bd..a120b5ba54db 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -724,8 +724,13 @@ void wbt_enable_default(struct gendisk *disk)
 	if (!blk_queue_registered(q))
 		return;
 
-	if (queue_is_mq(q) && enable)
+	if (queue_is_mq(q) && enable) {
 		wbt_init(disk);
+
+		mutex_lock(&q->debugfs_mutex);
+		blk_mq_debugfs_register_rq_qos(q);
+		mutex_unlock(&q->debugfs_mutex);
+	}
 }
 EXPORT_SYMBOL_GPL(wbt_enable_default);
 
-- 
2.39.2