On 4/19/07, Dmitriy V'jukov <dvyukov@gmail.com> wrote:

Peter Dimov wrote:

...

There's also a hidden ordinary store to the vtable pointer. In general, when passing an object X from thread A to thread B, you are expected to ensure the necessary visibility (and not the author of X). Usually the queue that

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you're using for message passing will contain the acquire/release pair in pop/push. Otherwise you wouldn't be able to pass ordinary objects between threads. Or to take a simple example:

Thank you very much. Now I understand. I underestimate the "pass to another thread" procedure, that must contain all necessary fences to ensure visibility of counter variable.

In particular, looking at your original example:

shared_count volatile* sc;

void thread1() { //... sc = new shared_count();

*** need a write barrier here, so that other thread doesn't see 'sc' set before its contents ***

//... }

void thread2() { //...

*** need a read barrier when reading 'sc', so that you don't see it before its contents ***

while (!sc) Sleep(1); sc->inc(); if (!sc->dec()) delete sc; //... }

Tony