Messages in this thread | | | Date | Thu, 5 Oct 2017 11:41:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays |
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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()?
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