linux-ext4 - Re: [RFC] ext3 freeze feature

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Message-Id: <004901c86925$7e3d1d60$41a8400a@bsd.tnes.nec.co.jp>
Date:	Thu, 7 Feb 2008 10:05:12 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Theodore Tso" <tytso@....EDU>
Cc:	<linux-ext4@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC] ext3 freeze feature

Hi,

> What you *could* do is to start putting processes to sleep if they
> attempt to write to the frozen filesystem, and then detect the
> deadlock case where the process holding the file descriptor used to
> freeze the filesystem gets frozen because it attempted to write to the
> filesystem --- at which point it gets some kind of signal (which
> defaults to killing the process), and the filesystem is unfrozen and
> as part of the unfreeze you wake up all of the processes that were put
> to sleep for touching the frozen filesystem.
> 
> The other approach would be to say, "oh well, the freeze ioctl is
> inherently dangerous, and root is allowed to himself in the foot, so
> who cares".  :-)

Currently the XFS freezer doesn't solve a deadlock automatically
and we rely on administrators for ensuring that the freezer will not
access the filesystem.
And even if the wrong freezer causes a deadlock, it can be solved
by other unfreeze process(unfreeze command).

So I don't think the freezer itself needs to solve the deadlock.
I think the timeout is effective for a unexpected deadlock
and the timeout extending feature is very useful
as Dmitri proposed.

Cheers, Takashi
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