[PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

> AFAIU the scheduler rq->lock is held while preemption is disabled.
> synchronize_sched() is used here to ensure that all pre-existing
> preempt-off critical sections have completed.
> 
> So saying that we use synchronize_sched() to synchronize with rq->lock
> would be stretching the truth a bit. It's actually only true because the
> scheduler holding the rq->lock is surrounded by a preempt-off
> critical section.

No, rq->lock is sufficient, note that rq->lock is a raw_spinlock_t which
implies !preempt. Yes, we also surround the rq->lock usage with a
slightly larger preempt_disable() section but that's not in fact
required for this.