raid5: reduce chance release_stripe() taking device_lock

release_stripe() is a place conf->device_lock is heavily contended. We take the
lock even stripe count isn't 1, which isn't required.
Signed-off-by: NShaohua Li <shli@fusionio.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -190,9 +190,8 @@ static int stripe_operations_active(struct stripe_head *sh)
 	       test_bit(STRIPE_COMPUTE_RUN, &sh->state);
 }
 
-static void __release_stripe(struct r5conf *conf, struct stripe_head *sh)
+static void do_release_stripe(struct r5conf *conf, struct stripe_head *sh)
 {
-	if (atomic_dec_and_test(&sh->count)) {
 	BUG_ON(!list_empty(&sh->lru));
 	BUG_ON(atomic_read(&conf->active_stripes)==0);
 	if (test_bit(STRIPE_HANDLE, &sh->state)) {
@@ -222,7 +221,12 @@ static void __release_stripe(struct r5conf *conf, struct stripe_head *sh)
 				md_wakeup_thread(conf->mddev->thread);
 		}
 	}
-	}
+}
+
+static void __release_stripe(struct r5conf *conf, struct stripe_head *sh)
+{
+	if (atomic_dec_and_test(&sh->count))
+		do_release_stripe(conf, sh);
 }
 
 static void release_stripe(struct stripe_head *sh)
@@ -230,9 +234,12 @@ static void release_stripe(struct stripe_head *sh)
 	struct r5conf *conf = sh->raid_conf;
 	unsigned long flags;
 
-	spin_lock_irqsave(&conf->device_lock, flags);
-	__release_stripe(conf, sh);
-	spin_unlock_irqrestore(&conf->device_lock, flags);
+	local_irq_save(flags);
+	if (atomic_dec_and_lock(&sh->count, &conf->device_lock)) {
+		do_release_stripe(conf, sh);
+		spin_unlock(&conf->device_lock);
+	}
+	local_irq_restore(flags);
 }
 
 static inline void remove_hash(struct stripe_head *sh)