Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 07, 2015 at 02:23:17PM +0100, Will Deacon wrote:
> Hi Peter,
> 
> Thanks for the headache ;)

Most welcome :-)

> > Does we want to go revert 12d560f4ea87 ("rcu,locking: Privatize
> > smp_mb__after_unlock_lock()") for that same reason?
> 
> I don't think we want a straight revert. smp_mb__after_unlock_lock could
> largely die if PPC strengthened its locks, whereas smp_mb__release_acquire
> is needed by quite a few architectures.

Fair enough, lets wait for the benchmark results from the PPC people
doing that.

> > > Documentation/memory-barriers.txt is updated to describe more clearly
> > > the ACQUIRE and RELEASE ordering in this area and to show an example of
> > > the new barrier in action.
> > 
> > The only nit I have is that if we revert the above it might be make
> > sense to more clearly call out the distinction between the two.
> 
> Right. Where I think we'd like to get to is:
> 
>  - RELEASE -> ACQUIRE acts as a full barrier if they operate on the same
>    variable and the ACQUIRE reads from the RELEASE
> 
>  - RELEASE -> ACQUIRE acts as a full barrier if they execute on the same
>    CPU and are interleaved with an smp_mb__release_acquire barrier.
> 
>  - RELEASE -> ACQUIRE ordering is transitive
> 
> [only the transitivity part is missing in this patch, because I lost
>  track of that discussion]
> 
> We could then use these same guarantees for UNLOCK -> LOCK in RCU,
> defining smp_mb__after_unlock_lock to be the same as
> smp_mb__release_acquire, but only applying to UNLOCK -> LOCK. That's a
> slight relaxation of how it's defined at the moment (and I guess would
> need some work on PPC?), but it keeps things consistent which is
> especially important as core locking primitives are ported over to the
> ACQUIRE/RELEASE primitives.
> 
> Thoughts?

/me like, although I'm too tired to see how those 3 rules combine to
something weaker than the current after_unlock_lock thing for PPC.