btrfs: switch to RCU for device traversal in btrfs_ioctl_dev_info

We don't need to use the mutex as we do not modify the devices nor the
list itself and just read some information:

does not change during device lifetime:
- devid
- uuid
- name (ie. the path)

may change in parallel to the ioctl call, but can lead only to reporting
inacurracy:
- bytes_used
- total_bytes

Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 64b9474..f1884c0 100644 (file)
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2775,7 +2775,6 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
 {
        struct btrfs_ioctl_dev_info_args *di_args;
        struct btrfs_device *dev;
-       struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
        int ret = 0;
        char *s_uuid = NULL;
 
@@ -2786,7 +2785,7 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
        if (!btrfs_is_empty_uuid(di_args->uuid))
                s_uuid = di_args->uuid;
 
-       mutex_lock(&fs_devices->device_list_mutex);
+       rcu_read_lock();
        dev = btrfs_find_device(fs_info, di_args->devid, s_uuid, NULL);
 
        if (!dev) {
@@ -2801,17 +2800,15 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
        if (dev->name) {
                struct rcu_string *name;
 
-               rcu_read_lock();
                name = rcu_dereference(dev->name);
                strncpy(di_args->path, name->str, sizeof(di_args->path));
-               rcu_read_unlock();
                di_args->path[sizeof(di_args->path) - 1] = 0;
        } else {
                di_args->path[0] = '\0';
        }
 
 out:
-       mutex_unlock(&fs_devices->device_list_mutex);
+       rcu_read_unlock();
        if (ret == 0 && copy_to_user(arg, di_args, sizeof(*di_args)))
                ret = -EFAULT;