Peter Zijlstra <pet...@infradead.org> writes: > On Thu, Jun 08, 2017 at 05:29:38PM +1000, Nicholas Piggin wrote: >> On Thu, 8 Jun 2017 08:54:00 +0200 >> Peter Zijlstra <pet...@infradead.org> wrote: >> > >> > 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
