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

[Steven Rostedt]

On Thu, 5 Oct 2017 22:19:15 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> 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.

Not only is preemption disabled (which is true for all spinlocks, at
least with non PREEMPT_RT), but the rq->lock must also only be taken
with interrupts disabled (for PREEMPT_RT as well). Because it can also
be taken in interrupt context.

-- Steve