Re: Perf hotplug lockup in v4.9-rc8

Paul E. McKenney
2016-12-22T16:33:41Z

On Thu, Dec 22, 2016 at 03:00:10PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 22, 2016 at 09:45:09AM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 12, 2016 at 01:42:28PM +0100, Peter Zijlstra wrote:
>
> > > > What are you trying to order here?
> > >
> > > I suppose something like this:
> > >
> > >
> > > CPU0 CPU1 CPU2
> > >
> > > (current == t)
> > >
> > > t->perf_event_ctxp[] = ctx;
> > > smp_mb();
> > > cpu = task_cpu(t);
> > >
> > > switch(t, n);
> > > migrate(t, 2);
> > > switch(p, t);
> > >
> > > ctx = t->perf_event_ctxp[]; // must not be NULL
> > >
> >
> > So I think I can cast the above into a test like:
> >
> > W[x] = 1 W[y] = 1 R[z] = 1
> > mb mb mb
> > R[y] = 0 W[z] = 1 R[x] = 0
> >
> > Where x is the perf_event_ctxp[], y is our task's cpu and z is our task
> > being placed on the rq of cpu2.
> >
> > See also commit: 8643cda549ca ("sched/core, locking: Document
> > Program-Order guarantees"), Independent of which cpu initiates the
> > migration between CPU1 and CPU2 there is ordering between the CPUs.
>
> I think that when we assume RCpc locks, the above CPU1 mb ends up being
> something like an smp_wmb() (ie. non transitive). CPU2 needs to do a
> context switch between observing the task on its runqueue and getting to
> switching in perf-events for the task, which keeps that a full mb.
>
> Now, if only this model would have locks in ;-)

Yeah, we are slow. ;-)

But you should be able to emulate them with xchg_acquire() and
smp_store_release().

Thanx, Paul

> > This would then translate into something like:
> >
> > C C-peterz
> >
> > {
> > }
> >
> > P0(int *x, int *y)
> > {
> > int r1;
> >
> > WRITE_ONCE(*x, 1);
> > smp_mb();
> > r1 = READ_ONCE(*y);
> > }
> >
> > P1(int *y, int *z)
> > {
> > WRITE_ONCE(*y, 1);
> > smp_mb();
>
> And this modified to: smp_wmb()
>
> > WRITE_ONCE(*z, 1);
> > }
> >
> > P2(int *x, int *z)
> > {
> > int r1;
> > int r2;
> >
> > r1 = READ_ONCE(*z);
> > smp_mb();
> > r2 = READ_ONCE(*x);
> > }
> >
> > exists
> > (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
>
> Still results in the same outcome.
>
> If however we change P2's barrier into a smp_rmb() it does become
> possible, but as said above, there's a context switch in between which
> implies a full barrier so no worries.
>
> Similar if I replace everything z with smp_store_release() and
> smp_load_acquire().
>
>
> Of course, its entirely possible the litmus test doesn't reflect
> reality, I still find it somewhat hard to write these things.
>