btrfs_free_extra_devids() is called only in the mount context which traverses through the fs_devices::devices and frees the orphan devices devices in the given %fs_devices if any. As the search for the orphan device is limited to fs_devices::devices so we don't need the global uuid_mutex.
There can't be any mount-point based ioctl threads in this context as the mount thread is not yet returned. But there can be the btrfs-control based scan ioctls thread which calls device_list_add(). Here in the mount thread the fs_devices::opened is incremented way before btrfs_free_extra_devids() is called and in the scan context the fs_devices which are already opened neither be freed or alloc-able at device_list_add(). But lets say you change the device-path and call the scan again, then scan would update the new device path and this operation could race against the btrfs_free_extra_devids() thread, which might be in the process of free-ing the same device. So synchronize it by using the device_list_mutex. This scenario is a very corner case, and practically the scan and mount are anyway serialized by the usage so unless the race is instrumented its very difficult to achieve. Signed-off-by: Anand Jain <anand.j...@oracle.com> Reviewed-by: David Sterba <dste...@suse.com> --- v3->v4: As we traverse through the seed device, fs_device gets updated with the child seed fs_devices, so make sure we use the same fs_devices pointer for the mutex_unlock as used for the mutex_lock. v2->v3: Update change log. (Currently device_list_add() is very lean on its device_list_mutex usage, a cleanup and fix is wip. Given the practicality of the above race condition this patch is good to merge). v1->v2: replace uuid_mutex with device_list_mutex instead of delete. change log updated. fs/btrfs/volumes.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 66f5ac5e8058..2166c5e7cc01 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -934,8 +934,9 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step) { struct btrfs_device *device, *next; struct btrfs_device *latest_dev = NULL; + struct btrfs_fs_devices *parent_fs_devices = fs_devices; - mutex_lock(&uuid_mutex); + mutex_lock(&parent_fs_devices->device_list_mutex); again: /* This is the initialized path, it is safe to release the devices. */ list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) { @@ -989,8 +990,7 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step) } fs_devices->latest_bdev = latest_dev->bdev; - - mutex_unlock(&uuid_mutex); + mutex_unlock(&parent_fs_devices->device_list_mutex); } static void free_device_rcu(struct rcu_head *head) -- 2.7.0 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html