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

Will Deacon
2016-09-01T11:19:15Z

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