Re: [boost] [interprocess] Atomic read and write.

From: Markus Schöpflin (markus.schoepflin_at_[hidden])
Date: 2007-10-10 17:35:38

Ion Gaztañaga schrieb:

> Markus Schöpflin escribió:
>> So it's not an issue of memory barriers, but of a hardware not being 
>> able to serialize concurrent r/w access to the same location (either 
>> register or memory address)?
> 
> Yes. With 32 bits there is no problem for current architectures, but 
> imagine we have a library for atomic 64 bit integers. Raw read and write 
> would not be atomic in 32 architectures because memory read/write would 
> be implemented with two memory accesses. We must also make sure that 
> cache and other issues maintain the atomicity.

OK, I think I understand now. Thank you and thanks to Daniel, who has 
been answering me in private mail.

I'll try to figure out whether Alpha hardware has this kind of atomicity 
build-in or whether it needs some special instructions for it.

Another question: the atomic compare and swap operation as described in 
the header file returns the old value of the memory location. CXX 
provides CAS as a primitive, but it doesn't return the old value, it 
simply returns true/false if the CAS has succeeded/failed. I have 
checked the usage of CAS in the interprocess library, and it looks like 
you don't actually need the return value of the CAS function, besides 
checking that the operation has succeeded. Am I right? Would you 
consider changing the interface for CAS to allow the usage of a compiler 
primitive which doesn't provide the value of the memory location?

> I'm not an atomic operation expert (even not a beginner). Functions were 
> inspired by apache portable runtime atomics, and there atomic read and 
> writes were wrapped. I just followed the example because I know that 
> 32bit operations were atomic in Intel32 but I also know that there are 
> systems (surely as used as Intel) that have not this property.

The header still contains the Apache license, I'm wondering if you 
simply can dual license it like you did? And isn't there a requirement 
that only boost licensed libraries are OK?

Thank you for your help,
Markus