Re: [PATCH] Btrfs: remove rcu_barrier in btrfs_close_devices

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Oct 11, 2017 at 03:41:23PM +0800, Anand Jain wrote:
> 
> 
> On 10/11/2017 02:11 PM, Anand Jain wrote:
> > 
> > 
> > On 10/11/2017 05:51 AM, Liu Bo wrote:
> > > It was introduced because btrfs used to do blkdev_put in a deferred
> > > work, now that btrfs has put blkdev in place, this rcu_barrier can be
> > > removed.
> 
>  On the 2nd thought, modprobe -r btrfs would still need rcu_barrier(), some
> where else outside of umount context ?

Thanks a lot for the comments.

modprobe -r btrfs will do btrfs_cleanup_fs_uuids(), where it cleanup
every %fs_devices on the list, but when we do btrfs_close_devices(), we
have replaced the devices on the list with dummy ones which only have
the same name and uuid, so modprobe -r btrfs will free those instead
of what we were using, this change won't cause a problem for it.

Thanks,

-liubo
> 
> Thanks, Anand
> 
> 
> >   Reviewed-by: Anand Jain <anand.jain@oracle.com>
> > 
> > Thanks, Anand
> > 
> > 
> > > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > > ---
> > >   fs/btrfs/volumes.c | 6 ------
> > >   1 file changed, 6 deletions(-)
> > > 
> > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > > index 0e8f16c..d983cea 100644
> > > --- a/fs/btrfs/volumes.c
> > > +++ b/fs/btrfs/volumes.c
> > > @@ -958,12 +958,6 @@ int btrfs_close_devices(struct btrfs_fs_devices
> > > *fs_devices)
> > >           __btrfs_close_devices(fs_devices);
> > >           free_fs_devices(fs_devices);
> > >       }
> > > -    /*
> > > -     * Wait for rcu kworkers under __btrfs_close_devices
> > > -     * to finish all blkdev_puts so device is really
> > > -     * free when umount is done.
> > > -     */
> > > -    rcu_barrier();
> > >       return ret;
> > >   }
> > >