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

From: Markus Schöpflin (markus.schoepflin_at_[hidden])
Date: 2007-10-10 15:29:41

Cory Nelson schrieb:
> On 10/10/07, Markus Schöpflin <markus.schoepflin_at_[hidden]> wrote:
>> Ion Gaztañaga schrieb:
>>
>>> Markus Schöpflin escribió:
>>>> Hello,
>>>>
>>>> this may be a silly question, but I have been wondering what the atomic
>>>> read and write primitives are actually supposed to do?
>>>>
>>>> I mean, from the readers or writers POV a read or write is always atomic,
>>>> isn't it? Am I missing something very obvious here?
>>> In some architectures, a 32 read might not be atomic (even if the read
>>> is aligned). I think that Intel system is always atomic. They are there
>>> for completeness, to support systems where read or write might not be
>>> atomic.
>> Sorry for being dense, but what exactly do you mean when you say a read
>> might not be atomic? Are you thinking of memory barriers here? Or is it
>> something else? Is there somewhere a definition of what atomic exactly
>> means here?
> 
> I believe he is referring to cache coherency - on x86 and x64, read
> and write are always atomic in the sense that a read will never see
> half of the old value and half of a write.

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)?

Markus