[PATCH V2 20/20] block: move wbt_enable_default() out of queue freezing from sched ->exit()

[Ming Lei <ming.lei@xxxxxxxxxx>]

scheduler's ->exit() is called with queue frozen and elevator lock is held, and
wbt_enable_default() can't be called with queue frozen, otherwise the
following lockdep warning is triggered:

	#6 (&q->rq_qos_mutex){+.+.}-{4:4}:
	#5 (&eq->sysfs_lock){+.+.}-{4:4}:
	#4 (&q->elevator_lock){+.+.}-{4:4}:
	#3 (&q->q_usage_counter(io)#3){++++}-{0:0}:
	#2 (fs_reclaim){+.+.}-{0:0}:
	#1 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}:
	#0 (&q->debugfs_mutex){+.+.}-{4:4}:

Fix the issue by moving wbt_enable_default() out of bfq's exit(), and
call it from elevator_change_done().

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
 block/bfq-iosched.c | 2 +-
 block/elevator.c    | 5 +++++
 block/elevator.h    | 1 +
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 40e4106a71e7..310ce1d8c41e 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -7211,7 +7211,7 @@ static void bfq_exit_queue(struct elevator_queue *e)
 
 	blk_stat_disable_accounting(bfqd->queue);
 	blk_queue_flag_clear(QUEUE_FLAG_DISABLE_WBT, bfqd->queue);
-	wbt_enable_default(bfqd->queue->disk);
+	set_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, &e->flags);
 
 	kfree(bfqd);
 }
diff --git a/block/elevator.c b/block/elevator.c
index 8652fe45a2db..378553fce5d8 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -687,8 +687,13 @@ int elevator_change_done(struct request_queue *q, struct elv_change_ctx *ctx)
 	int ret = 0;
 
 	if (ctx->old) {
+		bool enable_wbt = test_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT,
+				&ctx->old->flags);
+
 		elv_unregister_queue(q, ctx->old);
 		kobject_put(&ctx->old->kobj);
+		if (enable_wbt)
+			wbt_enable_default(q->disk);
 	}
 	if (ctx->new) {
 		ret = elv_register_queue(q, ctx->new, ctx->uevent);
diff --git a/block/elevator.h b/block/elevator.h
index 486be0690499..b14c611c74b6 100644
--- a/block/elevator.h
+++ b/block/elevator.h
@@ -122,6 +122,7 @@ struct elevator_queue
 
 #define ELEVATOR_FLAG_REGISTERED	0
 #define ELEVATOR_FLAG_DYING		1
+#define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT	2
 
 /* Holding context data for changing elevator */
 struct elv_change_ctx {
-- 
2.47.0