Q: schedule() and implied barriers on arm64

[Peter Zijlstra <peterz@infradead.org>]

Hi,

IIRC Paul relies on schedule() implying a full memory barrier with
strong transitivity for RCU.

If not, ignore this email.

If so, however, I suspect AARGH64 is borken and would need (just like
PPC):

#define smp_mb__before_spinlock()	smp_mb()

The problem is that schedule() (when a NO-OP) does:

	smp_mb__before_spinlock();
	LOCK rq->lock

	clear_bit()

	UNLOCK rq->lock

And nothing there implies a full barrier on AARGH64, since
smp_mb__before_spinlock() defaults to WMB, LOCK is an "ldaxr" or
load-acquire, UNLOCK is "stlrh" or store-release and clear_bit() isn't
anything.

Pretty much every other arch has LOCK implying a full barrier, either
because its strongly ordered or because it needs one for the ACQUIRE
semantics.