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

[Boqun Feng <boqun.feng@gmail.com>]

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On Thu, Sep 01, 2016 at 01:51:34PM +0200, Peter Zijlstra wrote:
> 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?
> 

My understanding is as Will stated, we don't provide this strong
gaurantee for spin_is_locked(). The reason is mostly because all(?)
uses of spin_is_locked() are not for correctness but for other purposes
like debug output.

> > 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.
> 

I think the hope is that, with herd or other tools, and a formal order
model, we can make more people understand this "counter-intuitive"
behavior and help them write correct and efficient code ;-)

> 
> Will, Boqun, did I get that right?
> 

Yep ;-)

Regards,
Boqun

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