Re: [PATCH 1/4] spinlock: Document memory barrier rules

Peter Zijlstra
2016-09-01T11:51:48Z

On Thu, Sep 01, 2016 at 01:04:26PM +0200, Manfred Spraul wrote:

> >So for both power and arm64, you can in fact model spin_unlock_wait()
> >as LOCK+UNLOCK.

> Is this consensus?

Dunno, but it was done to fix your earlier locking scheme and both
architectures where it matters have done so.

So I suppose that could be taken as consensus ;-)

> If I understand it right, the rules are:
> 1. spin_unlock_wait() must behave like spin_lock();spin_unlock();

>From a barrier perspective, yes I think so. Ideally the implementation
would avoid stores (which was the entire point of introducing that
primitive IIRC) if at all possible (not possible on ARM64/Power).

> 2. spin_is_locked() must behave like spin_trylock() ? spin_unlock(),TRUE :
> FALSE

Not sure on this one, That might be consistent, but I don't see the
ll/sc-nop in there. Will?

> 3. the ACQUIRE during spin_lock applies to the lock load, not to the store.

I think we can state that ACQUIRE on _any_ atomic only applies to the
LOAD not the STORE.

And we're waiting for that to bite us again before trying to deal with
it in a more generic manner; for now only the spinlock implementations
(specifically spin_unlock_wait) deal with it.

Will, Boqun, did I get that right?