linux-kernel - Re: [RFC] should VM_BUG_ON(cond) really evaluate cond

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Message-ID: <1319772566.6759.27.camel@deadeye>
Date:	Fri, 28 Oct 2011 04:29:26 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] should VM_BUG_ON(cond) really evaluate cond

On Fri, 2011-10-28 at 04:52 +0200, Eric Dumazet wrote:
> Le vendredi 28 octobre 2011 à 02:44 +0100, Ben Hutchings a écrit :
> 
> > Whether or not it needs to provide any ordering guarantee, atomic_read()
> > must never read more than once, and I think that requires the volatile
> > qualification.  It might be clearer to use the ACCESS_ONCE macro,
> > however.
> > 
> 
> Where this requirement comes from ?

That is the conventional behaviour of 'atomic' operations, and callers
may depend on it.

> Maybe then introduce atomic_read_once() for users really needing it :)
>
> ACCESS_ONCE will force the read/move instruction I try to avoid :(
[...]

I'm sure you can find some other way to avoid it.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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