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Re: [PATCH 3/3] Add timeout feature

To: Miklos Szeredi <miklos@xxxxxxxxxx>
Subject: Re: [PATCH 3/3] Add timeout feature
From: Pavel Machek <pavel@xxxxxxx>
Date: Wed, 9 Jul 2008 22:48:30 +0200
Cc: arjan@xxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, tytso@xxxxxxx, t-sato@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
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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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