btrfs: switch to RCU for device traversal in btrfs_ioctl_fs_info

We don't need to use the mutex as we do not modify the devices nor the
list itself and just read information about device counts.
Move copying fsid out of the protected section, not applicable to RCU
same as the rest of the retrieved information.

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

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index f1884c075cd3626c1526a9865bc44a4070c6bfa2..ab7ff3efcba6ebe20fbe237a1deb3bc3bbd8276c 100644 (file)
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2749,16 +2749,16 @@ static long btrfs_ioctl_fs_info(struct btrfs_fs_info *fs_info,
        if (!fi_args)
                return -ENOMEM;
 
-       mutex_lock(&fs_devices->device_list_mutex);
+       rcu_read_lock();
        fi_args->num_devices = fs_devices->num_devices;
-       memcpy(&fi_args->fsid, fs_info->fsid, sizeof(fi_args->fsid));
 
-       list_for_each_entry(device, &fs_devices->devices, dev_list) {
+       list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
                if (device->devid > fi_args->max_id)
                        fi_args->max_id = device->devid;
        }
-       mutex_unlock(&fs_devices->device_list_mutex);
+       rcu_read_unlock();
 
+       memcpy(&fi_args->fsid, fs_info->fsid, sizeof(fi_args->fsid));
        fi_args->nodesize = fs_info->nodesize;
        fi_args->sectorsize = fs_info->sectorsize;
        fi_args->clone_alignment = fs_info->sectorsize;