linux-kernel - Re: [PATCH 2/7] revoke: add f

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Message-Id: <200703091213.35691.dada1@cosmosbay.com>
Date:	Fri, 9 Mar 2007 12:13:35 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Pekka Enberg" <penberg@...helsinki.fi>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	hch@...radead.org, alan@...rguk.ukuu.org.uk, serue@...ibm.com
Subject: Re: [PATCH 2/7] revoke: add f_light flag for struct file

On Friday 09 March 2007 11:43, Pekka Enberg wrote:
> On 3/9/07, Eric Dumazet <dada1@...mosbay.com> wrote:
> > Cannot we use a flag in 'struct files_struct', set to one when the task
> > is mono-thread (at task creation in fact), and set to 0 when it creates a
> > new thread  (or when someone remotely access to this "struct
> > files_struct" in /proc/pid/fd/... )
>
> How does that work? fget_light() has a built-in assumption that as
> long as you don't share files_struct, it's okay not to take an extra
> reference as current is only one doing close(2) and revoke(2) changes
> that. So it's not really about being single-threaded or not.

I just dropped one (silly ?) idea and expected you made the hard work :)

Then just drop the fget_light() 'optimisation' and always take a reference 
(atomic on f_count) regardless of single-thread or not. Instead of dirtying 
f_light, just do the straightforward thing and be with it.

(that is : fget_light() = fget() = no more keeping fput_needed everywhere, and 
convoluted things in some dark sides of the kernel.

It will save some conditional branches and complexity, and you dont need this 
f_light thing.

>
> On 3/9/07, Eric Dumazet <dada1@...mosbay.com> wrote:
> > Also, the thing is racy.
>
> Aah, fget_light() indeed has a race window between fcheck_files() and
> set_f_light().

Yes, you see how hard it is to get this right.
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