Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Peter Zijlstra
2015-11-12T10:28:21Z

On Thu, Nov 12, 2015 at 03:14:51PM +0800, Boqun Feng wrote:
> Hmm.. probably incorrect.. because the ACQUIRE semantics of spin_lock()
> only guarantees that the memory operations following spin_lock() can't
> be reorder before the *LOAD* part of spin_lock() not the *STORE* part,
> i.e. the case below can happen(assuming the spin_lock() is implemented
> as ll/sc loop)
>
> spin_lock(&lock):
> r1 = *lock; // LL, r1 == 0
> o = READ_ONCE(object); // could be reordered here.
> *lock = 1; // SC
>
> This could happen because of the ACQUIRE semantics of spin_lock(), and
> the current implementation of spin_lock() on PPC allows this happen.

Urgh, you just _had_ to send an email like this, didn't you ;-)

I think AARGH64 does the same thing. They either use LDAXR/STXR, which
also places the ACQUIRE on the load, or use LDADDA (v8.1) which I
suspect does the same, but I'm not entirely sure how to decode these 8.1
atomics yet.

Let me think a little more on this..