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

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Message-ID: <20080709204829.GH11006@ucw.cz>
Date:	Wed, 9 Jul 2008 22:48:30 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	arjan@...radead.org, hch@...radead.org, tytso@....edu,
	t-sato@...jp.nec.com, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, mtk.manpages@...glemail.com
Subject: Re: [PATCH 3/3] Add timeout feature

On Wed 2008-07-09 09:08:07, Miklos Szeredi wrote:
> On Tue, 8 Jul 2008, Arjan van de Ven wrote:
> > I tihnk the idea there is
> > 
> > freeze . do the snapshot op . unfreeze . make backup of snapshot
> 
> Ah, so then my proposal would become
> 
>   run_frozen mountpoint do-snapshot
>   do-backup
>   release-snapshot
> 
> and if they are afraid of deadlocks they can just implement the
> timeout in userspace:
> 
>   run_frozen -t timeout mountpoint do-snapshot
> 
> 'run_frozen' can be a trivial 30 line app, that can be guaranteed not
> to deadlock.

Userland apps can be swapped out and need kernel memory allocations
during syscalls.

I bet even sleep(30) uses kmalloc internally.

So yes, even trivial applications can deadlock.

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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