Re: [PATCH 15/15] block: move wbt_enable_default() out of queue freezing from scheduler's ->exit()

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

On 4/10/25 7:00 PM, Ming Lei wrote:
> 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().

[...]

> diff --git a/block/elevator.c b/block/elevator.c
> index 1cc640a9db3e..9cf78db4d6a4 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -676,8 +676,13 @@ int elevator_change_done(struct request_queue *q, struct elev_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);
>  	}
wbt_enable_default is also called from ioc_qos_write and blk_register_queue 
with ->elevator_lock protection. So avoiding protection here doesn't seem
correct.

Thanks,
--Nilay