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SubjectRe: linux-next: manual merge of the rcu tree with the tip tree
On Tue, Aug 01, 2017 at 06:43:14AM -0700, Andy Lutomirski wrote:
> Anyway, can you document whatever property you require with a comment
> in switch_mm() or wherever you're finding that property so that future
> arch changes don't break it?

We need _a_ smp_mb after rq->curr store. x86 has plenty.

> > +static void membarrier_private_expedited(void)
> > +{
> > + int cpu;
> > + bool fallback = false;
> > + cpumask_var_t tmpmask;
> > +
> > + if (num_online_cpus() == 1)
> > + return;
> > +
> > + /*
> > + * Matches memory barriers around rq->curr modification in
> > + * scheduler.
> > + */
> > + smp_mb(); /* system call entry is not a mb. */
> > +
> > + /*
> > + * Expedited membarrier commands guarantee that they won't
> > + * block, hence the GFP_NOWAIT allocation flag and fallback
> > + * implementation.
> > + */
> > + if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
> > + /* Fallback for OOM. */
> > + fallback = true;
> > + }
> > +
> > + cpus_read_lock();
> > + for_each_online_cpu(cpu) {
> > + struct task_struct *p;
> > +
> > + /*
> > + * Skipping the current CPU is OK even through we can be
> > + * migrated at any point. The current CPU, at the point
> > + * where we read raw_smp_processor_id(), is ensured to
> > + * be in program order with respect to the caller
> > + * thread. Therefore, we can skip this CPU from the
> > + * iteration.
> > + */
> > + if (cpu == raw_smp_processor_id())
> > + continue;
> > + rcu_read_lock();
> > + p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> > + if (p && p->mm == current->mm) {
>
> I'm a bit surprised you're iterating all CPUs instead of just CPUs in
> mm_cpumask().

Because ARM64 doesn't set any bits at all in there.

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