On Wed, May 23, 2018 at 10:54:22AM +0800, Anand Jain wrote: > 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.jain@xxxxxxxxxx> Thanks, this explanation is much better and addresses the questions I have while reading the code. Reviewed-by: David Sterba <dsterba@xxxxxxxx> > --- > Currently device_list_add() is very lean on its device_list_mutex usage, > a cleanup and fix is wip. I also have a WIP patch to rewrite device_list_add. There were quite some changes around the device locking so I'd need to refresh it on top of current code first, it's not in a shape to be posted yet. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
