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

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Date:	Fri, 28 Oct 2011 04:37:53 -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 Thu, Oct 27, 2011 at 9:43 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> The only requirement of atomic_read() is that it must return value
> before or after an atomic_write(), not a garbled value.

The problem is that gcc *can* return a garbled value.

> In fact, if a compiler is stupid enough to issue two reads on following
> code :

The compiler really *can* be that "stupid". Except the code tends to
look like this:

   int value = atomic_read(&atomic_var);
   if (value > 10)
     return;
   .. do something with value ..

and gcc may decide - under register pressure, and in the absense of a
'volatile' - to read 'value' first once for that "> 10" check, and
then it drops the registers and instead of saving it on the stack
frame, it can decide to re-load it from atomic_var.

IOW, "value" could be two or more different values: one value when
testing, and *another* value in "do something with value".

This is why we have "ACCESS_ONCE()".

Whether atomics guarantee ACCESS_ONCE() semantics or not is not
entirely clear. But afaik, there is no way to tell gcc "access at
*most* once, and never ever reload".

                             Linus
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