On 05/28/2018 11:40 PM, David Sterba wrote:
On Mon, May 28, 2018 at 10:43:29PM +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.j...@oracle.com> --- (I didn't see this email in the mailing list, so trying again). 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.Well, now that I see the change, shouldn't we always hold the device_list_mutex of the fs_devices that's being processed? Ie. each time it's switched, the previous is unlocked and new one locked.
No David. That's because we organize seed device under its parent fs_devices ((fs_devices::seed)::seed)..so on, and they are a local cloned copy of the original seed fs_devices. So parent's fs_devices::device_list_mutex lock will suffice. Thanks, Anand
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