Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Feb 12, 2009 at 02:29:41PM -0500, Mathieu Desnoyers wrote:
> > 
> > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > [...]
> > > diff --git a/urcu.c b/urcu.c
> > > index f2aae34..a696439 100644
> > > --- a/urcu.c
> > > +++ b/urcu.c
> > > @@ -99,7 +99,8 @@ static void force_mb_single_thread(pthread_t tid)
> > >  	 * BUSY-LOOP.
> > >  	 */
> > >  	while (sig_done < 1)
> > > -		smp_rmb();	/* ensure we re-read sig-done */
> > > +		barrier();	/* ensure compiler re-reads sig-done */
> > > +				/* cache coherence guarantees CPU re-read. */
> > 
> > OK, this is where I think our points of view differ. Please refer to
> > http://lkml.org/lkml/2007/6/18/299.
> > 
> > Basically, cpu_relax() used in the Linux kernel has an
> > architecture-specific implementation which *could* include a smp_rmb()
> > if the architecture doesn't notice writes done by other CPUs. I think
> > Blackfin is the only architecture currently supported by the Linux
> > kernel which defines cpu_relax() as a smp_mb(), because it does not have
> > cache coherency.
> > 
> > Therefore, I propose that we create a memory barrier macro which is
> > defined as a 
> >   barrier()   when the cpu has cache coherency
> >   cache flush when the cpu does not have cache coherency and is
> >               compiled with smp support.
> > 
> > We could call that
> > 
> >   smp_wmc() (for memory-coherency or memory commit)
> >   smp_rmc()
> >   smp_mc()
> > 
> > It would be a good way to identify the location where data exchange
> > between memory and the local cache are is required in the algorithm.
> > What do you think ?
> 
> Actually the best way to do this would be:
> 
> 	while (ACCESS_ONCE(sig_done) < 1)
> 		continue;
> 

Interesting idea. Maybe we should define an accessor for the data write
too ?

But I suspect that in a lot of situations, what we will really want is
to do a bunch of read/writes, and only at a particular point do the
cache flush.

> If ACCESS_ONCE() needs to be made architecture-specific to make this
> really work on Blackfin, we should make that change.  And, now that
> you mention it, I have heard rumors that other CPU families can violate
> cache coherence in some circumstances.
> 
> So perhaps ACCESS_ONCE() becomes:
> 
> #ifdef CONFIG_ARCH_CACHE_COHERENT
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> #else /* #ifdef CONFIG_ARCH_CACHE_COHERENT */
> #define ACCESS_ONCE(x)     ({ \
> 				typeof(x) _________x1; \
> 				_________x1 = (*(volatile typeof(x) *)&(x)); \
> 				cpu_relax(); \

I don't think cpu_relax would be the correct primitive to use here. We
definitely don't want a "rep; nop;" or anything like this which _slows
down_ the access. It's just a different goal we are pursuing. So using
something like smp_rmc within the ACCESS_ONCE() macro in this case as I
proposed in the other mail still seems to make sense.

Mathieu

> 				(_________x1); \
> 				})
> #endif /* #else #ifdef CONFIG_ARCH_CACHE_COHERENT */
> 
> Seem reasonable?
> 
> 							Thanx, Paul
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68