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

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Date:   Thu, 1 Sep 2016 13:51:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     Will Deacon <will.deacon@....com>, benh@...nel.crashing.org,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 1/4] spinlock: Document memory barrier rules

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?