Messages in this thread | | | Date | Thu, 5 Oct 2017 07:55:13 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays |
| |
On Thu, Oct 05, 2017 at 11:41:14AM +0200, Peter Zijlstra wrote: > 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?
Your point being that both wait_for_completion() and complete() acquire and release the same lock? (Yes, I suspect that I was confusing this with wait_event() and wake_up(), just so you know.)
> And is not CPU0's QS reporting ordered against that complete()?
Mumble mumble mumble powerpc mumble mumble mumble...
OK, I will make this new memory barrier only execute for powerpc.
Or am I missing something else here?
Thanx, Paul
| |