btrfs: use RCU for quick device check in btrfs_init_new_device

When adding a new device there's a mandatory check to see if a device is
being duplicated to the filesystem it's added to. Since this is a
read-only operations not necessary to take device_list_mutex and can simply
make do with an rcu-readlock.

Using just RCU is safe because there won't be another device add delete
running in parallel as btrfs_init_new_device is called only from
btrfs_ioctl_add_dev.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 29c6f0154c62cb51e14398eb6877c735faf83702..5449e3899e672f3e88125428b1751eab76ceb031 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2515,16 +2515,15 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 
        filemap_write_and_wait(bdev->bd_inode->i_mapping);
 
-       mutex_lock(&fs_devices->device_list_mutex);
-       list_for_each_entry(device, &fs_devices->devices, dev_list) {
+       rcu_read_lock();
+       list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
                if (device->bdev == bdev) {
                        ret = -EEXIST;
-                       mutex_unlock(
-                               &fs_devices->device_list_mutex);
+                       rcu_read_unlock();
                        goto error;
                }
        }
-       mutex_unlock(&fs_devices->device_list_mutex);
+       rcu_read_unlock();
 
        device = btrfs_alloc_device(fs_info, NULL, NULL);
        if (IS_ERR(device)) {