On Thu, Jun 08, 2017 at 10:32:44AM +1000, Nicholas Piggin wrote: > On Wed, 07 Jun 2017 18:15:06 +0200 > Peter Zijlstra <pet...@infradead.org> wrote: > > > Now that the scheduler's rq->lock is RCsc and thus provides full > > transitivity between scheduling actions. And since we cannot migrate > > current, a task needs a switch-out and a switch-in in order to > > migrate, in which case the RCsc provides all the ordering we need. > > Hi Peter, > > I'm actually just working on removing this right now too, so > good timing. > > I think we can't "just" remove it, because it is required to order > MMIO on powerpc as well.
How is MMIO special? That is, there is only MMIO before we call into schedule() right? So the rq->lock should be sufficient to order that too. > > But what I have done is to comment that some other primitives are > already providing the hwsync for other, so we don't have to add > another one in _switch. 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. That said, I think you can already use the smp_mb__before_spinlock() as that is done with IRQs disabled, but its a more difficult argument. The rq->lock RCsc property should be more obvious.
