Subj : Re: Memory visibility and MS Interlocked instructions
To   : comp.programming.threads
From : Alexander Terekhov
Date : Sat Sep 03 2005 11:14 pm


Peter Dimov wrote:
[...]
> I find that very odd. Not the absence of atomic visibility per se,
> but...
> 
> P1: X = 1; #storestore; Y = 1;
> P2: if( Y == 1) { #loadstore; Z = 1; }

That's dependent store, no barrier needed.

> P3: if( Z == 1) { #loadload; assert( X == 1 ); }
> 
> If Z == 1 happens before X == 1, Z = 1 happens-before Z == 1, Y == 1
> happens-before Z = 1, Y = 1 happens-before Y == 1 and X = 1
> happens-before Y = 1, then X = 1 happens-before X == 1, right? 

Right. Under PC,

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

Z == 1 && X != 1 is false.

regards,
alexander.

.