Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays

From: Peter Zijlstra
Date: Thu Oct 05 2017 - 05:41:34 EST

On Wed, Oct 04, 2017 at 02:29:27PM -0700, Paul E. McKenney wrote:
> Consider the following admittedly improbable sequence of events:
> o RCU is initially idle.
> o Task A on CPU 0 executes rcu_read_lock().
> o Task B on CPU 1 executes synchronize_rcu(), which must
> wait on Task A:
> o Task B registers the callback, which starts a new
> grace period, awakening the grace-period kthread
> on CPU 3, which immediately starts a new grace period.
> o Task B migrates to CPU 2, which provides a quiescent
> state for both CPUs 1 and 2.
> o Both CPUs 1 and 2 take scheduling-clock interrupts,
> and both invoke RCU_SOFTIRQ, both thus learning of the
> new grace period.
> o Task B is delayed, perhaps by vCPU preemption on CPU 2.
> o CPUs 2 and 3 pass through quiescent states, which are reported
> to core RCU.
> o Task B is resumed just long enough to be migrated to CPU 3,
> and then is once again delayed.
> o Task A executes rcu_read_unlock(), exiting its RCU read-side
> critical section.
> o CPU 0 passes through a quiescent sate, which is reported to
> core RCU. Only CPU 1 continues to block the grace period.
> o CPU 1 passes through a quiescent state, which is reported to
> core RCU. This ends the grace period, and CPU 1 therefore
> invokes its callbacks, one of which awakens Task B via
> complete().
> o Task B resumes (still on CPU 3) and starts executing
> wait_for_completion(), which sees that the completion has
> already completed, and thus does not block. It returns from
> the synchronize_rcu() without any ordering against the
> end of Task A's RCU read-side critical section.
> It can therefore mess up Task A's RCU read-side critical section,
> in theory, anyway.

I'm not sure I follow, at the very least the wait_for_completion() does
an ACQUIRE such that it observes the state prior to the RELEASE as done
by complete(), no?

And is not CPU0's QS reporting ordered against that complete()?