Btrfs: drop unnecessary device lock

Drop device_list_mutex for the reader side  on clone_fs_devices and
btrfs_rm_device pathes since the fs_info->volume_mutex can ensure the device
list is not updated

btrfs_close_extra_devices is the initialized path, we can not add or remove
device at this time, so we can simply drop the mutex safely, like other
initialized function does(add_missing_dev, __find_device, __btrfs_open_devices
...).

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 6 insertions, 7 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index ee197ec28547..0b5ca2737268 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -406,7 +406,7 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
         fs_devices->latest_trans = orig->latest_trans;
         memcpy(fs_devices->fsid, orig->fsid, sizeof(fs_devices->fsid));
 
-        mutex_lock(&orig->device_list_mutex);
+        /* We have held the volume lock, it is safe to get the devices. */
         list_for_each_entry(orig_dev, &orig->devices, dev_list) {
                 device = kzalloc(sizeof(*device), GFP_NOFS);
                 if (!device)
@@ -429,10 +429,8 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
                 device->fs_devices = fs_devices;
                 fs_devices->num_devices++;
         }
-        mutex_unlock(&orig->device_list_mutex);
         return fs_devices;
 error:
-        mutex_unlock(&orig->device_list_mutex);
         free_fs_devices(fs_devices);
         return ERR_PTR(-ENOMEM);
 }
@@ -443,7 +441,7 @@ int btrfs_close_extra_devices(struct btrfs_fs_devices *fs_devices)
 
         mutex_lock(&uuid_mutex);
 again:
-        mutex_lock(&fs_devices->device_list_mutex);
+        /* This is the initialized path, it is safe to release the devices. */
         list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
                 if (device->in_fs_metadata)
                         continue;
@@ -463,7 +461,6 @@ again:
                 kfree(device->name);
                 kfree(device);
         }
-        mutex_unlock(&fs_devices->device_list_mutex);
 
         if (fs_devices->seed) {
                 fs_devices = fs_devices->seed;
@@ -1242,14 +1239,16 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
 
                 device = NULL;
                 devices = &root->fs_info->fs_devices->devices;
-                mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
+                /*
+                 * It is safe to read the devices since the volume_mutex
+                 * is held.
+                 */
                 list_for_each_entry(tmp, devices, dev_list) {
                         if (tmp->in_fs_metadata && !tmp->bdev) {
                                 device = tmp;
                                 break;
                         }
                 }
-                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                 bdev = NULL;
                 bh = NULL;
                 disk_super = NULL;