[PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[Elijah Wright]

in dd_finish_request(), per_prio points to a rq->elv.priv[0], which could be
free memory if an in-flight requests completes after its associated scheduler
has been freed

Signed-off-by: Elijah Wright <git@elijahs.space>
---
 block/mq-deadline.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 2edf1cac06d5..4d7b21b144d3 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -751,13 +751,15 @@ static void dd_finish_request(struct request *rq)
 {
 	struct dd_per_prio *per_prio = rq->elv.priv[0];
 
-	/*
-	 * The block layer core may call dd_finish_request() without having
-	 * called dd_insert_requests(). Skip requests that bypassed I/O
-	 * scheduling. See also blk_mq_request_bypass_insert().
-	 */
-	if (per_prio)
-		atomic_inc(&per_prio->stats.completed);
+	if (rq->q->elevator) {
+		/*
+		* The block layer core may call dd_finish_request() without having
+		* called dd_insert_requests(). Skip requests that bypassed I/O
+		* scheduling. See also blk_mq_request_bypass_insert().
+		*/
+		if (per_prio)
+			atomic_inc(&per_prio->stats.completed);
+	}
 }
 
 static bool dd_has_work_for_prio(struct dd_per_prio *per_prio)
-- 
2.43.0

Re: [PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[Bart Van Assche]

On 6/17/25 1:56 PM, Elijah Wright wrote:
> in dd_finish_request(), per_prio points to a rq->elv.priv[0], which could be
> free memory if an in-flight requests completes after its associated scheduler
> has been freed
> 
> Signed-off-by: Elijah Wright <git@elijahs.space>
> ---
>   block/mq-deadline.c | 16 +++++++++-------
>   1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 2edf1cac06d5..4d7b21b144d3 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -751,13 +751,15 @@ static void dd_finish_request(struct request *rq)
>   {
>   	struct dd_per_prio *per_prio = rq->elv.priv[0];
>   
> -	/*
> -	 * The block layer core may call dd_finish_request() without having
> -	 * called dd_insert_requests(). Skip requests that bypassed I/O
> -	 * scheduling. See also blk_mq_request_bypass_insert().
> -	 */
> -	if (per_prio)
> -		atomic_inc(&per_prio->stats.completed);
> +	if (rq->q->elevator) {
> +		/*
> +		* The block layer core may call dd_finish_request() without having
> +		* called dd_insert_requests(). Skip requests that bypassed I/O
> +		* scheduling. See also blk_mq_request_bypass_insert().
> +		*/
> +		if (per_prio)
> +			atomic_inc(&per_prio->stats.completed);
> +	}
>   }

The warnings in dd_exit_sched() will be triggered if dd_finish_request()
is ever called with rq->q->elevator == NULL.

If this can happen, it should be fixed in the block layer core instead
of in the mq-deadline scheduler.

Thanks,

Bart.

Re: [PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[Elijah Wright]

I see. would it be possible to detach the elevator from the queue in 
elevator_exit instead?

On 6/17/2025 3:25 PM, Bart Van Assche wrote:
> On 6/17/25 1:56 PM, Elijah Wright wrote:
>> in dd_finish_request(), per_prio points to a rq->elv.priv[0], which 
>> could be
>> free memory if an in-flight requests completes after its associated 
>> scheduler
>> has been freed
>>
>> Signed-off-by: Elijah Wright <git@elijahs.space>
>> ---
>>   block/mq-deadline.c | 16 +++++++++-------
>>   1 file changed, 9 insertions(+), 7 deletions(-)
>>
>> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
>> index 2edf1cac06d5..4d7b21b144d3 100644
>> --- a/block/mq-deadline.c
>> +++ b/block/mq-deadline.c
>> @@ -751,13 +751,15 @@ static void dd_finish_request(struct request *rq)
>>   {
>>       struct dd_per_prio *per_prio = rq->elv.priv[0];
>> -    /*
>> -     * The block layer core may call dd_finish_request() without having
>> -     * called dd_insert_requests(). Skip requests that bypassed I/O
>> -     * scheduling. See also blk_mq_request_bypass_insert().
>> -     */
>> -    if (per_prio)
>> -        atomic_inc(&per_prio->stats.completed);
>> +    if (rq->q->elevator) {
>> +        /*
>> +        * The block layer core may call dd_finish_request() without 
>> having
>> +        * called dd_insert_requests(). Skip requests that bypassed I/O
>> +        * scheduling. See also blk_mq_request_bypass_insert().
>> +        */
>> +        if (per_prio)
>> +            atomic_inc(&per_prio->stats.completed);
>> +    }
>>   }
> 
> The warnings in dd_exit_sched() will be triggered if dd_finish_request()
> is ever called with rq->q->elevator == NULL.
> 
> If this can happen, it should be fixed in the block layer core instead
> of in the mq-deadline scheduler.
> 
> Thanks,
> 
> Bart.

Re: [PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[Bart Van Assche]

On 6/17/25 3:52 PM, Elijah Wright wrote:
> I see. would it be possible to detach the elevator from the queue in 
> elevator_exit instead?

elevator_switch() is called with the request queue frozen.
q_usage_counter is dropped after the blk_mq_finish_request() call
returned. So what you described in your patch description can't happen.

Bart.