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

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Message-ID: <20111028012521.GF25795@one.firstfloor.org>
Date:	Fri, 28 Oct 2011 03:25:21 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.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, Oct 28, 2011 at 03:19:21AM +0200, Eric Dumazet wrote:
> In commit 4e60c86bd9e (gcc-4.6: mm: fix unused but set warnings)
> Andi forced VM_BUG_ON(cond) to evaluate cond, even if CONFIG_DEBUG_VM is
> not set :
> 
> #ifdef CONFIG_DEBUG_VM
> #define VM_BUG_ON(cond) BUG_ON(cond)
> #else
> #define VM_BUG_ON(cond) do { (void)(cond); } while (0)
> #endif

Eventually the warnings were disabled in the Makefile.
So it would be reasonable to just revert that patch now, at least
for VM_BUG_ON, if it costs performance.

> 
> So maybe a fix would be to introduce an atomic_read_stable() variant ?
> 
> static inline int atomic_read_stable(const atomic_t *v)
> {
> 	return v->counter;
> }

Seems reasonable too. In fact we usually should have memory barriers
for this anyways which obsolete the volatile.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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