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

Boqun Feng
2016-09-01T14:05:23Z

[-- Attachment #1: Type: text/plain, Size: 1890 bytes --]

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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 473 bytes --]