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 f1884c0..ab7ff3e 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;