[dm-devel] Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

[David Chinner <dgc sgi com>]

On Wed, Nov 08, 2006 at 12:01:09AM +0000, Alasdair G Kergon wrote:
> On Wed, Nov 08, 2006 at 12:42:02AM +0100, Rafael J. Wysocki wrote:
> > On Wednesday, 8 November 2006 00:18, Eric Sandeen wrote:
> > > But, how is a stampede of fs-freezers -supposed- to work?  I could
> > > imagine something like a freezer count, and the filesystem is only
> > > unfrozen after everyone has thawed?  Or should only one freezer be
> > > active at a time... which is what we have now I guess.
> > I think it shouldn't be possible to freeze an fs more than once.
> 
> In device-mapper today, the only way to get more than one freeze on the
> same device is to use xfs and issue xfs_freeze before creating an lvm snapshot
> (or issuing the dmsetup equivalent), and at the moment we tell people not to do
> that any more.

But it's trivial to detect this condition - if (sb->s_frozen != SB_UNFROZEN)
then the filesystem is already frozen and you shouldn't try to freeze
it again. It's simple to do, and the whole problem then just goes away....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group