Re: Behaviour of smp_mb__{before,after}_spin* and acquire/release

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

On Tue, Jan 20, 2015 at 09:34:43AM +0000, Peter Zijlstra wrote:
> On Tue, Jan 13, 2015 at 04:33:54PM +0000, Will Deacon wrote:
> > I started dusting off a series I've been working to implement a relaxed
> > atomic API in Linux (i.e. things like atomic_read(v, ACQUIRE)) but I'm
> > having trouble making sense of the ordering semantics we have in mainline
> > today:
> 
> >   2. Does smp_mb__after_unlock_lock order smp_store_release against
> >      smp_load_acquire? Again, Documentation/memory-barriers.txt puts
> >      these operations into the RELEASE and ACQUIRE classes respectively,
> >      but since smp_mb__after_unlock_lock is a NOP everywhere other than
> >      PowerPC, I don't think this is enforced by the current code. 
> 
> Yeah, wasn't Paul going to talk to Ben about that? PPC is the only arch
> that has the weak ACQUIRE/RELEASE for its spinlocks.

Indeed, and I'd love to kill that, especially as its really confusing
when we have other ACQUIRE/RELEASE functions (like your smp_* accessors)
that do need explicit barriers for general RELEASE->ACQUIRE ordering.

If people start using smp_mb__after_unlock_lock for *that*, then other
architectures will need to implement it as a barrier and penalise their
spinlocks in doing so.

> >      Most
> >      architectures follow the pattern used by asm-generic/barrier.h:
> > 
> >        release: smp_mb(); STORE
> >        acquire: LOAD; smp_mb();
> > 
> >      which doesn't provide any release -> acquire ordering afaict.
> 
> Only when combined on the same address, if the LOAD observes the result
> of the STORE we can guarantee the rest of the ordering. And if you
> build a locking primitive with them (or circular lists or whatnot) you
> have that extra condition.
> 
> But yes, I see your argument that this implementation is weak like the
> PPC.

I'm absolutely fine with that, I'd just like to make sure that it's
documented so that people don't use smp_mb__after_unlock_lock() to
order smp_store_release -> smp_load_acquire.

I'll have a crack at a Documentation patch if you don't beat me to it...

Will