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

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Date:   Thu, 1 Sep 2016 12:19:15 +0100
From:   Will Deacon <will.deacon@....com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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:
> If I understand it right, the rules are:
> 1. spin_unlock_wait() must behave like spin_lock();spin_unlock();
> 2. spin_is_locked() must behave like spin_trylock() ? spin_unlock(),TRUE :
> FALSE

I don't think spin_is_locked is as strong as all that. On arm64 and ppc,
it's just smp_mb(); followed by a check on the lock value. It can't be
used for the same sorts of inter-CPU synchronisation that spin_unlock_wait
provides.

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

Correct. This is already documented for things like cmpxchg.

> sem.c and nf_conntrack.c need only rule 1 now, but I would document the rest
> as well, ok?
> 
> I'll update the patches.

Please CC me!

Will