Messages in this thread | | | From | Joel Fernandes <> | Date | Tue, 01 May 2018 15:23:52 +0000 | Subject | Re: [PATCH RFC v5 5/6] tracepoint: Make rcuidle tracepoint callers use SRCU |
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On Tue, May 1, 2018 at 8:20 AM Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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
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