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block/elevator: reduce the critical section

PATCH In review Hui Su 2 participants 3 messages Oct 19, 2020 Block layer

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Hui Su

author
#0 lore raw

PATCH 1/1 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(-)
block/elevator.c +6 -8
index 293c5c81397a..727902b31954 100644
@@ -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;
 }
 
 /*
Signature · 1 line
2.25.1
maintainer
#1 lore raw
Quoted 19 lines · 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 <[email protected]>
---
 block/elevator.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)
block/elevator.c +0 -0
index 293c5c81397a..727902b31954 100644
@@ -151,11 +151,11 @@ static struct elevator_type *elevator_get(struct request_queue *q,
 		spin_lock(&elv_list_lock);

block/elevator.c static struct elevator_type *elevator_get(struct request_queue *q, · line 160

Quoted 10 lines
block/elevator.c +1 -1
 		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();
Signature · 1 line
Jens Axboe
maintainer
#2 lore raw
In reply to Hui Su
Quoted 17 lines · Hui Su
block/elevator.c +2 -3
@@ -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);

block/elevator.c static struct elevator_type *elevator_get_default(struct request_queue *q) · line 651

Quoted 10 lines
block/elevator.c +3 -4
 
-	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.

Signature · 1 line
Jens Axboe