On Thu, Aug 02, 2018 at 05:29:12PM +0800, Anand Jain wrote:

On 08/01/2018 10:29 PM, David Sterba wrote:

quoted

On Thu, Jul 26, 2018 at 02:53:32PM +0800, Anand Jain wrote:

quoted

From: Anand Jain <redacted>

%fs_devices can be free-ed by btrfs_free_stale_devices() when the
close_fs_devices() drops fs_devices::opened to zero, but close_fs_devices
tries to access the %fs_devices again without the device_list_mutex.

Fix this by bringing the %fs_devices access with in the device_list_mutex.

AFAICS this cannot happen anymore because the two calls are serialized
by the uuid_mutex. But this was not the case when syzbot reported the
problem where your patch would apply.

The parallell access to opened and device list cannot happen when:

* btrfs_scan_one_device that wants to call btrfs_free_stale_devices
* btrfs_close_devices calls close_fs_devices

Fixed by the series:

btrfs: lift uuid_mutex to callers of btrfs_scan_one_device
btrfs: lift uuid_mutex to callers of btrfs_open_devices
btrfs: lift uuid_mutex to callers of btrfs_parse_early_options
btrfs: reorder initialization before the mount locks uuid_mutex
btrfs: fix mount and ioctl device scan ioctl race

If there's a race I don't see, please describe in more detail.

  Right. There is no race with the uuid_mutex patches as above.

  And I just found this- can we make close be consistent with its
  open part.
  btrfs_open_devices() hold device_list_mutex before the update to
  fs_devices::opened. So close_fs_device() could do the same, and be
  theoretically correct.

Or it can be the other way around, to push the device_list_mutex only
around the list_sort and open_fs_devices like:

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -1144,15 +1144,15 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 
        lockdep_assert_held(&uuid_mutex);
 
-       mutex_lock(&fs_devices->device_list_mutex);
        if (fs_devices->opened) {
                fs_devices->opened++;
                ret = 0;
        } else {
+               mutex_lock(&fs_devices->device_list_mutex);
                list_sort(NULL, &fs_devices->devices, devid_cmp);
                ret = open_fs_devices(fs_devices, flags, holder);
+               mutex_unlock(&fs_devices->device_list_mutex);
        }
-       mutex_unlock(&fs_devices->device_list_mutex);
 
        return ret;
 }