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

Hi Alasdair, Eric and Dave,

On Thu, Jul 03, 2008 at 01:47:10PM +0100, Alasdair G Kergon wrote:
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.

Seconded - that was also my primary objection to the timeout code.

I will consider removing the timeout.

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.

Yes, I've seen that reported a number of times.

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

Agreed, though I'd modify the definition of that case to be "remain
frozen until the last thaw occurs". That has the advantage that
it's relatively simple to implement with just a counter...

I agree this idea.
But I have one concern. When device-mapper's freeze follows FIFREEZE,
can device-mapper freeze only device-mapper's part correctly?
And when device-mapper's thaw follows FITHAW,
can device-mapper thaw only device-mapper's part?

Cheers, Takashi
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