[cpp-threads] Causality on more than two processors, write atomicity

[Alexander Terekhov]

On 9/3/05, Peter Dimov <pdimov at mmltd.net> wrote:
> There is a discussion in comp.programming.threads, in which the following
> example:
> 
> P1: Y = 1;
> P2: if( Y == 1 ) { Z = 1; }
> P3: if( Z == 1 ) { assert( Y == 1 ); }
> 
> is claimed to legitimately assert under "processor consistency" - the memory
> model that the x86 CPUs are supposed to implement, and under all platforms
> that don't exhibit write atomicity (even when all loads are ld.acq and all
> stores are st.rel).

Itanic defines "remote write atomicity" for st.rel to WB memory only.

> 
> I'm still struggling with this "discovery". How does it affect us? It seems
> to me that this makes ld.acq/st.rel impossible to specify in a
> happens-before framework 

That probably depends on the framework. Y = 1 (and Z = 1) are 
writes in progress, so to speak.

P1: Y = 1; Y = 2;
P2: if( Y == 2 ) { Z = 1; }
P3: if( Z == 1 ) { assert( Y >= 1 ); }

won't abort legitimately.

regards,
alexander.