Re: [patch] Re: using long instead of atomic_t when only set/read is required

Pavel Machek
2008-03-03T17:36:44Z

Hi!

> > Ok, so linux actually atomicity of long?
^~-- assumes should be here.

> No it doesn't. And even if it did you couldn't use long for this because
> atomic_t also ensures the points operations complete are defined. You
> might just about get away with volatile long * objects on x86 for simple
> assignments but for anything else gcc can and will generate code to
> update values whichever way it feels best - which includes turning
>
> long *x = a + b;
>
> into
>
> *x = a;
> *x += b;

Ok, I can understand the gcc side. But do we actually run on an
architecture where

long *x;

*x = 0;

racing with

*x = 0x12345678;

can produce

*x == 0x12340000;

or something like that? I'm told RCU relies on architectures not doing
this, and I'd like to get this clarified.
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html