Re: [PATCH] btrfs: do not return EBUSY on concurrent subvolume mounts

[David Sterba <dsterba@suse.cz>]

On Wed, Apr 27, 2016 at 04:22:17PM -0700, Liu Bo wrote:
> On Wed, Apr 27, 2016 at 05:14:36PM +0200, David Sterba wrote:
> > A user reported mount failures with EBUSY during boot, there's root
> > partition and many subvolumes, mounted via /etc/fstab.
> > 
> > The failure depends on timing, when multiple subvolumes reach the code
> > between superblock creation in RO mode, while the subvolumes are RW.
> > This discrepancy leads to EBUSY and the code has been there since ages.
> > 
> > If the subvolumes are mounted after a short delay, there's no EBUSY.
> > There's no missing locking, the supreblock creation is atomic and the
> > error code seems to be just artificial. We support different RO/RW
> > mounts in mount_subvol and do the relevant adjustments if the flags do
> > not match.
> 
> Looks good to me.
> 
> Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
> 
> But What I'm worrying about is that mount_subvol() will help subvolume
> mounting get correct mount flags by calling btrfs_remount(), and
> btrfs_remount() can remove sb->s_flags's MS_RDONLY.  In all syscall cases
> it's ok as we have mnt->mnt_flags, but for btrfs's add_dev ioctl, we
> don't check mnt_want_write_file() while btrfs's rm_dev ioctl does the
> check, should we add that for add_dev ioctl?

That's right, I have patches to fix that and will send them.