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

[Will Deacon <will.deacon@arm.com>]

Hi Peter,

Thanks for the headache ;)

On Wed, Oct 07, 2015 at 01:19:15PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2015 at 11:59:28AM +0100, Will Deacon wrote:
> > As much as we'd like to live in a world where RELEASE -> ACQUIRE is
> > always cheaply ordered and can be used to construct UNLOCK -> LOCK
> > definitions with similar guarantees, the grim reality is that this isn't
> > even possible on x86 (thanks to Paul for bringing us crashing down to
> > Earth).
> > 
> > This patch handles the issue by introducing a new barrier macro,
> > smp_mb__release_acquire, that can be placed between a RELEASE and a
> > subsequent ACQUIRE operation in order to upgrade them to a full memory
> > barrier. At the moment, it doesn't have any users, so its existence
> > serves mainly as a documentation aid.
> 
> 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.

> > 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?

Will