Hi!
quoted
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
On Monday, 3 of March 2008, Pavel Machek wrote:
Hi!
quoted
quoted
Ok, so linux actually atomicity of long?
^~-- assumes should be here.
quoted
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?
Well something like this could happen, in theory, on a "32-bit" architecture
with a 16-bit bus. In that case, one can imagine, the first word of the
first write may be sent through the bus immediately followed by the first
word of the second write, followed by the second word of the second
write and by the second word of the first write, in this order.
I'm told RCU relies on architectures not doing this, and I'd like to get this
clarified.
Yes, it would be good to know that for sure.
Rafael