On Wed, May 07, 2014 at 07:29:16PM +0100, Will Deacon wrote:
>   better_spin_lock(atomic_t *lock)
>   {
> 	/*
> 	 * Atomic add to lock with acquire semantics, returning original
> 	 * value.
> 	 */
> 	int old = atomic_xchg_add(ACQUIRE, lock, 1 << TICKET_SHIFT);
> 	if ((old << TICKET_SHIFT) == (old & (TICKET_MASK << TICKET_SHIFT)))
> 		return; /* Got the lock */
> 
> 	while (smp_load_acquire((u16 *)lock) != (old >> TICKET_SHIFT))
> 		cpu_relax();
>   }

So xchg_add and atomic_add_return() are pretty much an identity map
because the add operation is reversible. The other popular name for this
operation is fetch_add() fwiw.

In any case, something that's been brewing in the back of my mind is an
ATOMIC_OP() and ATOMIC_RET_OP() macro construct that takes a lambda
function (expr-stmt is I think the closes we get in C) and either
generates the appropriate ll/sc loop or a cmpxchg loop, depending on
arch.

Its fairly sad that many of the generic atomic operations we do with
cmpxchg loops where ll/sc archs can do better using their ll/sc
primitive and I just feel there must be a semi-sane way to express all
that, despite C :-)

</rant>

That also reminds me, I should send round 2 of the arch atomic cleanups
(I've got the patches), and write changelogs for round 3, and start on
the patches for round 4 :-)