btrfs: fix get set label blocking against balance

btrfs_ioctl_get_fslabel() and btrfs_ioctl_set_fslabel()
used root->fs_info->volume_mutex mutex which caused operations
like balance to block set/get label operation until its
completion and generally balance operation takes a long
time to complete, so it will be annoying to the user when
cli appears hung

also this patch will add a bit of optimization within
the btrfs_ioctl_get_falabel() function.

v1->v2:
   use fs_info->super_lock instead of uuid_mutex

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index f856c97952db..556b3d5b18da 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -4042,18 +4042,22 @@ out:
 static int btrfs_ioctl_get_fslabel(struct file *file, void __user *arg)
 {
         struct btrfs_root *root = BTRFS_I(file_inode(file))->root;
-        const char *label = root->fs_info->super_copy->label;
-        size_t len = strnlen(label, BTRFS_LABEL_SIZE);
+        size_t len;
         int ret;
+        char label[BTRFS_LABEL_SIZE];
+
+        spin_lock(&root->fs_info->super_lock);
+        memcpy(label, root->fs_info->super_copy->label, BTRFS_LABEL_SIZE);
+        spin_unlock(&root->fs_info->super_lock);
+
+        len = strnlen(label, BTRFS_LABEL_SIZE);
 
         if (len == BTRFS_LABEL_SIZE) {
                 pr_warn("btrfs: label is too long, return the first %zu bytes\n",
                         --len);
         }
 
-        mutex_lock(&root->fs_info->volume_mutex);
         ret = copy_to_user(arg, label, len);
-        mutex_unlock(&root->fs_info->volume_mutex);
 
         return ret ? -EFAULT : 0;
 }
@@ -4082,18 +4086,18 @@ static int btrfs_ioctl_set_fslabel(struct file *file, void __user *arg)
         if (ret)
                 return ret;
 
-        mutex_lock(&root->fs_info->volume_mutex);
         trans = btrfs_start_transaction(root, 0);
         if (IS_ERR(trans)) {
                 ret = PTR_ERR(trans);
                 goto out_unlock;
         }
 
+        spin_lock(&root->fs_info->super_lock);
         strcpy(super_block->label, label);
+        spin_unlock(&root->fs_info->super_lock);
         ret = btrfs_end_transaction(trans, root);
 
 out_unlock:
-        mutex_unlock(&root->fs_info->volume_mutex);
         mnt_drop_write_file(file);
         return ret;
 }