Re: [boost] [lockfree] review

Subject: Re: [boost] [lockfree] review
From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2011-08-24 18:24:13

Dave Abrahams wrote:
[...]
> >   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();
> >
> > and thus result in reodering i = 1 and j = 2.
> >
> > With C++11 default atomics (SC) for i and j such reodering is
> > prohibited.
> 
> As it is with C++11 mutexes, IIUC.  

Mutexes need only *acquire-release* semantics coupled with RMW(s) to
result in *SC* for associated race-free data, as observed by another
thread(s).

C++11 default atomics are *SC* without mutexes (lockfree and all that
stuff)... *SC* is more expansive than *acquire-release* in terms of sync
on platforms such as:

> If you have contrary evidence, please point to it.

http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html
(PowerPC)

I mean:

  Load SC: hwsync; ld; cmp; bc; isync
  Store SC: hwsync; st

alternative mappings:

  Load SC:  ld; hwsync
  Store SC: lwsync; st; hwsync

Note that *acquire-release* does NOT need *hw*sync:

  Load Acquire: ld; cmp; bc; isync
  Store Release: lwsync; st

regards,
alexander.