RE: [boost] Re: Work in progress list? And how about a singleton?

From: scleary_at_[hidden]
Date: 2001-11-16 09:31:29

> From: Kevin Cline [mailto:kcline_at_[hidden]]
> 
> The double-checked locking idiom is not safe on multi-processor systems.
> There was considerable discussion of this in comp.lang.c++.moderated
> a couple of years ago, but eventually everyone saw that it was unsafe.
> 
> On p. 148 of Alexandrescu's "Modern C++ Design" he nicely summarizes
> the discussion:
> 
>      Very experienced multithreaded programmers know that even the
>      Double-Checked Locking system ... is not always correct 
> in practice.
>      In certain symmetric multiprocessor environments ... the 
> writes are
>      committed to the main memory in bursts, rather than one by one.
>      ... Due to this rearranging of writes, the memory as seen by one
>      processor might look as if the operations are not performed
>      in the correct order by another processor...  the Double-Checked
>      Locking pattern is known to be defective for such systems.

OK -- I had to go look up these discussions (I don't have group access :( ).

First, some scope: this applies only to relaxed-memory model symmetric
multiprocessor systems.  

Second: I don't see how these systems could possibly claim to be conforming
anyway -- I have the usual questions about the guarantee of observable
behaviour, side effects, and sequence points -- which I'm sure you've heard
before.  My question: why can't these guarantees be used to prevent problems
with double-checked locking?  I did not find any threads where these were
fully addressed.

BTW, please respond privately; this really isn't a Boost discussion.

        -Steve