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SubjectRe: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU
On Wed, 11 Jul 2018 14:56:47 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:
> > static inline void tracepoint_synchronize_unregister(void)
> > {
> > + synchronize_srcu(&tracepoint_srcu);
> > synchronize_sched();
> > }
>
> Given you below do call_rcu_sched() and then call_srcu(), isn't the
> above the wrong way around?

Good catch!

release_probes()
call_rcu_sched()
---> rcu_free_old_probes() queued

tracepoint_synchronize_unregister()
synchronize_srcu(&tracepoint_srcu);
< finishes right away >
synchronize_sched()
--> rcu_free_old_probes()
--> srcu_free_old_probes() queued

Here tracepoint_synchronize_unregister() returned before the srcu
portion ran.


>
> Also, does the above want to be barrier instead of synchronize, so as to
> guarantee completion of the callbacks.

Not sure what you mean here.

-- Steve

>
> > +static void srcu_free_old_probes(struct rcu_head *head)
> > {
> > kfree(container_of(head, struct tp_probes, rcu));
> > }
> >
> > +static void rcu_free_old_probes(struct rcu_head *head)
> > +{
> > + call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);
> > +}
> > +
> > static inline void release_probes(struct tracepoint_func *old)
> > {
> > if (old) {
> > struct tp_probes *tp_probes = container_of(old,
> > struct tp_probes, probes[0]);
> > + /*
> > + * Tracepoint probes are protected by both sched RCU and SRCU,
> > + * by calling the SRCU callback in the sched RCU callback we
> > + * cover both cases. So let us chain the SRCU and sched RCU
> > + * callbacks to wait for both grace periods.
> > + */
> > call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> > }
> > }

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