[PATCH] fix try_module_get race in elevator_find

This patch removes try_module_get race in elevator_find.
try_module_get should always be called with the spinlock protecting
what the module init/cleanup routines register/unregister to held. In
the case of elevators, we should be holding elv_list to avoid it going
away between spin_unlock_irq and try_module_get.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 14 insertions, 10 deletions

diff --git a/drivers/block/elevator.c b/drivers/block/elevator.c
index 98f0126a2deb..4144f30d82a9 100644
--- a/drivers/block/elevator.c
+++ b/drivers/block/elevator.c
@@ -97,7 +97,6 @@ static struct elevator_type *elevator_find(const char *name)
         struct elevator_type *e = NULL;
         struct list_head *entry;
 
-        spin_lock_irq(&elv_list_lock);
         list_for_each(entry, &elv_list) {
                 struct elevator_type *__e;
 
@@ -108,7 +107,6 @@ static struct elevator_type *elevator_find(const char *name)
                         break;
                 }
         }
-        spin_unlock_irq(&elv_list_lock);
 
         return e;
 }
@@ -120,12 +118,15 @@ static void elevator_put(struct elevator_type *e)
 
 static struct elevator_type *elevator_get(const char *name)
 {
-        struct elevator_type *e = elevator_find(name);
+        struct elevator_type *e;
 
-        if (!e)
-                return NULL;
-        if (!try_module_get(e->elevator_owner))
-                return NULL;
+        spin_lock_irq(&elv_list_lock);
+
+        e = elevator_find(name);
+        if (e && !try_module_get(e->elevator_owner))
+                e = NULL;
+
+        spin_unlock_irq(&elv_list_lock);
 
         return e;
 }
@@ -153,11 +154,15 @@ static char chosen_elevator[16];
 
 static void elevator_setup_default(void)
 {
+        struct elevator_type *e;
+
         /*
          * check if default is set and exists
          */
-        if (chosen_elevator[0] && elevator_find(chosen_elevator))
+        if (chosen_elevator[0] && (e = elevator_get(chosen_elevator))) {
+                elevator_put(e);
                 return;
+        }
 
 #if defined(CONFIG_IOSCHED_AS)
         strcpy(chosen_elevator, "anticipatory");
@@ -555,10 +560,9 @@ void elv_unregister_queue(struct request_queue *q)
 
 int elv_register(struct elevator_type *e)
 {
+        spin_lock_irq(&elv_list_lock);
         if (elevator_find(e->elevator_name))
                 BUG();
-
-        spin_lock_irq(&elv_list_lock);
         list_add_tail(&e->list, &elv_list);
         spin_unlock_irq(&elv_list_lock);