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

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....


Dave Chinner
Principal Engineer
SGI Australian Software Group

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