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

From: Linus Torvalds
Date: Fri Oct 28 2011 - 07:38:18 EST

On Thu, Oct 27, 2011 at 9:43 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> 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)
.. 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".

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