Re: [boost] [lockfree] review

Subject: Re: [boost] [lockfree] review
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2011-08-27 12:10:05

On Fri, Aug 26, 2011 at 2:26 PM, Dave Abrahams <dave_at_[hidden]> wrote:
>
> on Fri Aug 26 2011, Alexander Terekhov <terekhov-AT-web.de> wrote:
>
>> Dave Abrahams wrote:
>> [...]
>>> IIUC, using mutexes in the implementation of C++0x atomics has always
>>> been considered an available choice for implementors. ...
>>
>> Lock-free atomics can not be implemented using mutexes. Lock-free SC
>> atomics need a heavy-weight 'hwsync' on PowerPC/PPC (mutexes don't need
>> 'hwsync').
>
> Thanks for educating me.
>
>> Nobody needs atomics implemented using mutexes because locking can be
>> done more efficiently on higher level.
>
> Well, the idea is to allow code using atomics to run at all (if
> suboptimally) on platforms that don't support hardware atomics for the
> data type in question.
>
> --
> Dave Abrahams
> BoostPro Computing
> http://www.boostpro.com
>

I think you are trying to highlight an important point here, and I
agree with you (that it is important and needs to be looked into - not
that you said that specifically...).

The idea in the standard is that

atomic<Foo> foo;

is suppose to work for any Foo, I think.  If Foo is too big to be
atomic by the CPU, then a mutex is to be used inside the atomic<>.

And that should have the same semantics as atomic<int>.

So my first question is, is the above true?
Second is whether that is a problem.  Note that in:

>> >   mi.lock();
 >> >   i = 1;
 >> >   mi.unlock();
 >> >
 >> >   mj.lock();
 >> >   j = 2;
 >> >   mj.unlock();
 >> >
 >> > can be transformed to
 >> >
 >> >   multi_lock(mi, mj); // deadlock free
 >> >   j = 2;
 >> >   i = 1;
 >> >   mi.unlock();
 >> >   mj.unlock();

There is no "synchronizes with" (standard's wording) because
synch-with requires thread 2 to *read* the value written into the
atomic by thread 1.  We have no reads here - no if statement - so
reordering doesn't matter.
I don't think the standard disallows the reordering of i and j, even
if they are atomics.

Tony