Re: [boost] [atomic] Help understanding consume order

Subject: Re: [boost] [atomic] Help understanding consume order
From: Tim Blechmann (tim_at_[hidden])
Date: 2014-06-02 03:45:46

hi andrey,

> I'm reviewing (again) Boost.Atomic code and struggling to understand the 
> consume order and in particular what should it mean on architectures other 
> than DEC Alpha.
> 
> I read the explanation here:
> 
> http://en.cppreference.com/w/cpp/atomic/memory_order
> 
> but the point eludes me. Take ARM for example and the explanation in the 
> "Release-Consume ordering" section. The producer thread allocates the string 
> and stores the pointer with a release operation, so that the pointer, the 
> string contents and the 'data' integer are visible to other threads.
> 
> Now the consumer thread reads the pointer with a consume operation. According 
> to the explanation in the article, on ARM the consume operation need not issue 
> any specific fences to be able to use the pointer and the string body. In that 
> case, the consume operation becomes equivalent to relaxed (plus prohibiting 
> compiler optimizations). But is there a guarantee that the string body will be 
> visible to the consumer? Shouldn't the consume operation be promoted to 
> acquire instead?

the alpha can speculatively load an memory region. the consume semantics
prevents issues a memory barrier to make sure that the memory is read
after the atomic pointer is written to. so from my understanding, on arm
and x86, relaxed has the same guarantee than consume ...

would be great if someone could confirm this, though.

tim