2020-10-19 16:47:32

by Hui Su

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

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 <[email protected]>
---
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

2020-10-19 16:52:25

by Jens Axboe

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

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 <[email protected]>
> ---
> 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

2020-10-19 17:26:20

by Jens Axboe

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

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