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

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Date:	Fri, 4 Jul 2008 21:08:09 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Eric Sandeen" <sandeen@...hat.com>,
	"Alasdair G Kergon" <agk@...hat.com>,
	"Dave Chinner" <david@...morbit.com>
Cc:	<dm-devel@...hat.com>, <linux-fsdevel@...r.kernel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<viro@...IV.linux.org.uk>, <linux-ext4@...r.kernel.org>,
	<xfs@....sgi.com>, "Christoph Hellwig" <hch@...radead.org>,
	<linux-kernel@...r.kernel.org>, <axboe@...nel.dk>,
	<mtk.manpages@...glemail.com>
Subject: 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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