linux-ext4 - Re: [dm-devel] Re: [PATCH 3/3] Add timeout feature

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Date:	Thu, 3 Jul 2008 13:47:10 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	Christoph Hellwig <hch@...radead.org>, linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, viro@...IV.linux.org.uk,
	dm-devel@...hat.com, xfs@....sgi.com, linux-kernel@...r.kernel.org,
	mtk.manpages@...glemail.com, axboe@...nel.dk
Subject: Re: [dm-devel] Re: [PATCH 3/3] Add timeout feature

On Thu, Jul 03, 2008 at 09:11:05PM +0900, Takashi Sato wrote:
> If the freezer accesses the frozen filesystem and causes a deadlock,
> the above ideas can't solve it

But you could also say that if the 'freezer' process accesses the frozen
filesystem and deadlocks then that's just a bug and that userspace code
should be fixed and there's no need to introduce the complexity of a
timeout parameter.

> >Similarly if a device-mapper device is involved, how should the following
> >sequence behave - A, B or C?
> >
> >1. dmsetup suspend (freezes)
> >2. FIFREEZE
> >3. FITHAW
> >4. dmsetup resume (thaws)
> [...]
> >C:
> > 1 succeeds, freezes
> > 2 fails, remains frozen
> > 3 fails (because device-mapper owns the freeze/thaw), remains frozen
> > 4 succeeds, thaws
> 
> I think C is appropriate and the following change makes it possible.
> How do you think?

The point I'm trying to make here is:
  Under what real-world circumstances might multiple concurrent freezing
  attempts occur, and which of A, B or C (or other variations) would be
  the most appropriate way of handling such situations?

A common example is people running xfs_freeze followed by an lvm command
which also attempts to freeze the filesystem.

I can see a case for B or C, but personally I prefer A:

> > 1 succeeds, freezes
> > 2 succeeds, remains frozen
> > 3 succeeds, remains frozen
> > 4 succeeds, thaws

Alasdair
-- 
agk@...hat.com
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