lkml.org 
[lkml]   [2011]   [Oct]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond
On Thu, Oct 27, 2011 at 9:43 PM, Eric Dumazet <eric.dumazet@gmail.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


\
 
 \ /
  Last update: 2011-10-28 13:41    [W:0.343 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site