Re: [PATCH [RT] 05/14] rearrange rt_spin_lock sleep

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2008-02-22 at 08:35 -0500, Steven Rostedt wrote:

> > My assumption is that the xchg() (inside update_current()) acts as an
> > effective wmb().  If xchg() does not have this property, then this code
> > is broken and patch 6/14 should also add a:
> >
> >
> > +               smp_wmb();
> 
> I believe that the wmb would be needed. I doubt that xchg on all archs
> would force any ordering of reads and writes. It only needs to guarantee the
> atomic nature of the data exchange. I don't see any reason that it would
> imply any type of memory barrier.

Documentation/memory-barriers.txt states:

Any atomic operation that modifies some state in memory and returns information
about the state (old or new) implies an SMP-conditional general memory barrier
(smp_mb()) on each side of the actual operation (with the exception of
explicit lock operations, described later).  These include:

        xchg();