Btrfs: avoid unnecessary scrub workers allocation

We only allocate scrub workers if we pass all the necessary
checks, for example, there are no operation in progress.

Besides, move mutex lock protection outside of scrub_workers_get()
/scrub_workers_put().

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/scrub.c

@@ -2784,7 +2784,6 @@ static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
 {
 	int ret = 0;
 
-	mutex_lock(&fs_info->scrub_lock);
 	if (fs_info->scrub_workers_refcnt == 0) {
 		if (is_dev_replace)
 			btrfs_init_workers(&fs_info->scrub_workers, "scrub", 1,
@@ -2814,21 +2813,17 @@ static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
 	}
 	++fs_info->scrub_workers_refcnt;
 out:
-	mutex_unlock(&fs_info->scrub_lock);
-
 	return ret;
 }
 
 static noinline_for_stack void scrub_workers_put(struct btrfs_fs_info *fs_info)
 {
-	mutex_lock(&fs_info->scrub_lock);
 	if (--fs_info->scrub_workers_refcnt == 0) {
 		btrfs_stop_workers(&fs_info->scrub_workers);
 		btrfs_stop_workers(&fs_info->scrub_wr_completion_workers);
 		btrfs_stop_workers(&fs_info->scrub_nocow_workers);
 	}
 	WARN_ON(fs_info->scrub_workers_refcnt < 0);
-	mutex_unlock(&fs_info->scrub_lock);
 }
 
 int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
@@ -2889,23 +2884,18 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		return -EINVAL;
 	}
 
-	ret = scrub_workers_get(fs_info, is_dev_replace);
-	if (ret)
-		return ret;
 
 	mutex_lock(&fs_info->fs_devices->device_list_mutex);
 	dev = btrfs_find_device(fs_info, devid, NULL, NULL);
 	if (!dev || (dev->missing && !is_dev_replace)) {
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		scrub_workers_put(fs_info);
 		return -ENODEV;
 	}
-	mutex_lock(&fs_info->scrub_lock);
 
+	mutex_lock(&fs_info->scrub_lock);
 	if (!dev->in_fs_metadata || dev->is_tgtdev_for_dev_replace) {
 		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		scrub_workers_put(fs_info);
 		return -EIO;
 	}
 
@@ -2916,10 +2906,17 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		btrfs_dev_replace_unlock(&fs_info->dev_replace);
 		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		scrub_workers_put(fs_info);
 		return -EINPROGRESS;
 	}
 	btrfs_dev_replace_unlock(&fs_info->dev_replace);
+
+	ret = scrub_workers_get(fs_info, is_dev_replace);
+	if (ret) {
+		mutex_unlock(&fs_info->scrub_lock);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		return ret;
+	}
+
 	sctx = scrub_setup_ctx(dev, is_dev_replace);
 	if (IS_ERR(sctx)) {
 		mutex_unlock(&fs_info->scrub_lock);
@@ -2957,10 +2954,10 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 
 	mutex_lock(&fs_info->scrub_lock);
 	dev->scrub_device = NULL;
+	scrub_workers_put(fs_info);
 	mutex_unlock(&fs_info->scrub_lock);
 
 	scrub_free_ctx(sctx);
-	scrub_workers_put(fs_info);
 
 	return ret;
 }