Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from _switch

Peter Zijlstra [off-list ref] writes:

On Thu, Jun 08, 2017 at 05:29:38PM +1000, Nicholas Piggin wrote:

quoted

On Thu, 8 Jun 2017 08:54:00 +0200
Peter Zijlstra [off-list ref] wrote:

quoted

Right, so this patch relies on the smp_mb__before_spinlock ->
smp_mb__after_spinlock conversion that makes the rq->lock RCsc and
should thus provide the required SYNC for migrations.

AFAIKS either one will do, so long as there is a hwsync there. The
point is just that I have added some commentary in the generic and
powerpc parts to make it clear we're relying on that behavior of
the primitive. smp_mb* is not guaranteed to order MMIO, it's just
that it does on powerpc.

I'm not particularly happy with the generic comment; I don't feel we
should care that PPC is special here.

I think it'd be nice if there was *some* comment on the two uses of
smp_mb__after_spinlock(), it's fairly subtle, but I don't think it needs
to mention PPC specifically.


If we have:

arch/powerpc/include/asm/barrier.h:
+/*
+ * This must resolve to hwsync on SMP for the context switch path. See
+ * _switch.
+ */
 #define smp_mb__after_spinlock()   smp_mb()


And then something in _switch() that says "we rely on the
smp_mb__after_spinlock() in the scheduler core being a hwsync", that
should probably be sufficient.

cheers