Subject: Re: bit fields && data tearing

From: "H. Peter Anvin" <[email-masked]>
Date: Mon, 08 Sep 2014 19:30:29 -0700

On 09/08/2014 03:39 PM, James Bottomley wrote:
> 
> I don't understand what you mean by "pass each other".  Atomicity
> guarantees are not ordering guarantees in a SMP environment.  The
> guarantee is that if you follow the rules when two CPUs update the same
> natural width aligned object simultaneously using the same primitive,
> the result is either one or the other of their updates.  Which one wins
> (the ordering) isn't defined.
> 

I'm trying to figure out why it would possibly make a difference in any
kind of sane system if gcc fuses accesses.

Assuming bigendian for the moment, I would expect that if CPU 1 does a
write of 0x01020304 to address 0 and CPU 2 does a write of 0x0506 to
address 2, that the end result would be either 0x01020304 or 0x01020506.
 Similarly, I would expect that if these operations are both done on the
same CPU in that order, that the result would unambiguously be 0x01020506.

I would strongly suspect an architecture which does not provide those
guarantees is an outlier.

	-hpa