[PATCH] block/elevator: reduce the critical section

[PATCH] block/elevator: reduce the critical section

[Hui Su]

1.reduce the critical section in elevator_get().
2.reduce the critical section in elevator_get_by_features().
3.remove the found variable.

the elv_list_lock is used to protect the elv_list,
and the operations of elevator_type does not need
to be protected.

Signed-off-by: Hui Su <sh_def@163.com>
---
 block/elevator.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 293c5c81397a..727902b31954 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -151,11 +151,11 @@ static struct elevator_type *elevator_get(struct request_queue *q,
 		spin_lock(&elv_list_lock);
 		e = elevator_find(name, q->required_elevator_features);
 	}
+	spin_unlock(&elv_list_lock);
 
 	if (e && !try_module_get(e->elevator_owner))
 		e = NULL;
 
-	spin_unlock(&elv_list_lock);
 	return e;
 }
 
@@ -633,23 +633,21 @@ static struct elevator_type *elevator_get_default(struct request_queue *q)
  */
 static struct elevator_type *elevator_get_by_features(struct request_queue *q)
 {
-	struct elevator_type *e, *found = NULL;
+	struct elevator_type *e = NULL;
 
 	spin_lock(&elv_list_lock);
-
 	list_for_each_entry(e, &elv_list, list) {
 		if (elv_support_features(e->elevator_features,
 					 q->required_elevator_features)) {
-			found = e;
 			break;
 		}
 	}
+	spin_unlock(&elv_list_lock);
 
-	if (found && !try_module_get(found->elevator_owner))
-		found = NULL;
+	if (e && !try_module_get(e->elevator_owner))
+		e = NULL;
 
-	spin_unlock(&elv_list_lock);
-	return found;
+	return e;
 }
 
 /*
-- 
2.25.1

Re: [PATCH] block/elevator: reduce the critical section

[Jens Axboe]

On 10/19/20 10:42 AM, Hui Su wrote:
> 1.reduce the critical section in elevator_get().
> 2.reduce the critical section in elevator_get_by_features().
> 3.remove the found variable.
> 
> the elv_list_lock is used to protect the elv_list,
> and the operations of elevator_type does not need
> to be protected.
> 
> Signed-off-by: Hui Su <sh_def@163.com>
> ---
>  block/elevator.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 293c5c81397a..727902b31954 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -151,11 +151,11 @@ static struct elevator_type *elevator_get(struct request_queue *q,
>  		spin_lock(&elv_list_lock);
>  		e = elevator_find(name, q->required_elevator_features);
>  	}
> +	spin_unlock(&elv_list_lock);
>  
>  	if (e && !try_module_get(e->elevator_owner))
>  		e = NULL;
>  
> -	spin_unlock(&elv_list_lock);
>  	return e;
>  }

What happens for:

	A					B
	spin_unlock(&elv_list_lock);
						rmmod -> elv_unregister();
	try_module_get();

-- 
Jens Axboe

Re: [PATCH] block/elevator: reduce the critical section

[Jens Axboe]

On 10/19/20 10:42 AM, Hui Su wrote:
> @@ -633,23 +633,21 @@ static struct elevator_type *elevator_get_default(struct request_queue *q)
>   */
>  static struct elevator_type *elevator_get_by_features(struct request_queue *q)
>  {
> -	struct elevator_type *e, *found = NULL;
> +	struct elevator_type *e = NULL;
>  
>  	spin_lock(&elv_list_lock);
> -
>  	list_for_each_entry(e, &elv_list, list) {
>  		if (elv_support_features(e->elevator_features,
>  					 q->required_elevator_features)) {
> -			found = e;
>  			break;
>  		}
>  	}
> +	spin_unlock(&elv_list_lock);
>  
> -	if (found && !try_module_get(found->elevator_owner))
> -		found = NULL;
> +	if (e && !try_module_get(e->elevator_owner))
> +		e = NULL;
>  
> -	spin_unlock(&elv_list_lock);
> -	return found;
> +	return e;
>  }

This looks wrong as well. If we don't match the elevator, then we just
return the last one. Or maybe even a totally invalid entry, depending on
how the list iteration works.


-- 
Jens Axboe