On Fri, Nov 20 2009 at 1:00pm -0500,
Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
> On Thu, 19 Nov 2009, Mike Snitzer wrote:
>
> > Permit in-use snapshot exception data to be 'handed over' from one
> > snapshot instance to another. This is a pre-requisite for patches
> > that allow the changes made in a snapshot device to be merged back into
> > its origin device and also allows device resizing.
> >
> > The basic call sequence is:
> >
> > dmsetup load new_snapshot (referencing the existing in-use cow device)
> > - the ctr code detects that the cow is already in use and allows the
> > two snapshot target instances to be linked together
> > dmsetup suspend original_snapshot
> > dmsetup resume new_snapshot
> > - the new_snapshot becomes live, and if anything now tries to access
> > the original one it will receive -EIO
> > dmsetup remove original_snapshot
> >
> > (There can only be two snapshot targets referencing the same cow device
> > simultaneously.)
> >
> > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Acked-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> > + down_read(&_origins_lock);
> > + (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest);
> > + if (snap_src && snap_dest) {
> > + down_write_nested(&snap_src->lock, SINGLE_DEPTH_NESTING);
> > + down_write(&snap_dest->lock);
>
> I'd suggest to use the "_nested" version for the nested lock, it is a
> convention.
>
> down_write(&snap_src->lock);
> down_write_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
OK, I've tested this to work fine. Thanks for spotting this.
Alasdair, are you OK with editing v9 of this patch to include Mikulas'
change? And also add his 'Signed-off-by'?
Or should I send v10?
Mike
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