Re: [PATCH RFC v5 5/6] tracepoint: Make rcuidle tracepoint callers use SRCU

[Joel Fernandes]

On Tue, May 1, 2018 at 8:20 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
wrote:
[...]
> > > > > --- a/kernel/tracepoint.c
> > > > > +++ b/kernel/tracepoint.c
> > > > > @@ -31,6 +31,9 @@
> > > > >  extern struct tracepoint * const __start___tracepoints_ptrs[];
> > > > >  extern struct tracepoint * const __stop___tracepoints_ptrs[];
> > > > >
> > > > > +DEFINE_SRCU(tracepoint_srcu);
> > > > > +EXPORT_SYMBOL_GPL(tracepoint_srcu);
> > > > > +
> > > > >  /* Set to 1 to enable tracepoint debug output */
> > > > >  static const int tracepoint_debug;
> > > > >
> > > > > @@ -67,11 +70,16 @@ static inline void *allocate_probes(int count)
> > > > >     return p == NULL ? NULL : p->probes;
> > > > >  }
> > > > >
> > > > > -static void rcu_free_old_probes(struct rcu_head *head)
> > > > > +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);
> > > >
> > > > Hmm, is it OK to call call_srcu() from a call_rcu() callback? I
guess
> > > > it would be.
> >
> > > It is perfectly legal, and quite a bit simpler than setting something
> > > up to wait for both to complete concurrently.
> >
> > Cool. Also in this case if we call both in sequence, then I felt there
> > could be a race to free the old data since both callbacks would try to
do
> > the same thing. The same thing being freeing of the same set of old
probes
> > which would need some synchronization between the 2 callbacks. With the
> > chaining, since the ordering is assured there wouldn't be a question of
> > such a race. I could add this reasoning to the changelog as well.

> Actually, as long as you have a solid happens-before between both of the
> callbacks and the freeing, you are in good shape.  A release-acquire would
> work fine, as would a lock acquired in both callbacks and then acquired
> (and possibly released) before the free.

Got it, thanks. For now, if its Ok with you and others, I will leave it in
the chained configuration. I also feel this is temporary since in the
future if we switch to single rcu mechanism for tracepoints (srcu), then we
could do with just a single callback.

thanks,

- Joel