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

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Date:	Fri, 28 Oct 2011 05:40:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	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: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond

On Fri, Oct 28, 2011 at 5:19 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> "Sane interfaces" are important. Insane interfaces lead to bugs.

Qutie frankly, if I do "atomic_read()", I do expect to get a single
value. If I don't get a single value, but some mixture of two values,
I'd personally go

  wtf, what does that "atomic" mean in "atomic_read()"?

and I think that's a reasonable wtf to ask.

That said, as mentioned, I don't know of any way to tell gcc "at most once".

Hmm.

Except perhaps using inline asm. Something like this might work:

  static inline int atomic_read(const atomic_t *v)
  {
       int val;
       asm("":"=r" (val):"0" (v->value));
       return val;
  }

(totally untested, but you get the idea: use a non-volatile asm to
make sure that gcc doesn't think it can re-load the value).

That's the trick we use in asmlinkage_protect() and a couple of other
places. It *should* make gcc able to optimize the value away entirely
if it isn't used, but will stop gcc from doing the reload magic.

Does that work for the test-case with VM_BUG_ON()?

                          Linus
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