Messages in this thread Patch in this message | | | Subject | Re: rcu-bh design | From | Joel Fernandes <> | Date | Fri, 4 May 2018 16:20:41 -0700 |
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On 05/04/2018 03:49 PM, Paul E. McKenney wrote: >> Yes just one more ;-). I am trying to write a 'probetorture' test inspired >> by RCU torture that whacks the tracepoints in various scenarios. One of the >> things I want to do is verify the RCU callbacks are queued and secondly, >> they are executed. Just to verify that the garbage collect was done and >> we're not leaking the function probe table (not that I don't have >> confidence in the chained callback technique which you mentioned, but it >> would be nice to assure this mechanism is working for tracepoints). >> >> Is there a way to detect this given a reference to srcu_struct? Mathieu and >> we were chatting about srcu_barrier which is cool but that just tells me >> that if there was a callback queued, it would have executed after the >> readers were done. But not whether something was queued. > > Suppose that you are queuing an RCU callback that in turn queues an SRCU > callback on my_srcu_struct, like this: > > void my_rcu_callback(struct rcu_head *rhp) > { > p = container_of(rhp, struct my_struct, my_rcu_head); > > free_it_up_or_down(p); > } > > void my_srcu_callback(struct rcu_head *rhp) > { > call_rcu(rhp, my_rcu_callback); > } > > call_srcu(&my_srcu_struct, &p->my_rcu_head, my_srcu_callback); > > Then to make sure that any previously submitted callback has been fully > processed, you do this: > > rcu_barrier(); > srcu_barrier(&my_srcu_struct); > > Of course if you queue in the opposite order, like this: > > void my_srcu_callback(struct rcu_head *rhp) > { > p = container_of(rhp, struct my_struct, my_rcu_head); > > free_it_up_or_down(p); > } > > void my_rcu_callback(struct rcu_head *rhp) > { > call_srcu(&my_srcu_struct, &p->my_rcu_head, my_srcu_callback); > } > > call_rcu(rhp, my_rcu_callback); > > Then you must wait in the opposite order: > > rcu_barrier(); > srcu_barrier(&my_srcu_struct); > > Either way, the trick is that the first *_barrier() call cannot return > until after all previous callbacks have executed, which means that by that > time the callback is enqueued for the other flavor of {S,}RCU. So the > second *_barrier() call must wait for the callback to be completely done, > through both flavors of {S,}RCU. > > So after executing the pair of *_barrier() calls, you know that the > callback is no longer queued. > > Does that cover it, or am I missing a turn in here somewhere?
Yes, that covers some of it. Thanks a lot. Btw, I was also thinking I want to check that srcu received a callback queuing request as well (not just completion but also queuing).
Say that the tracepoint code is buggy and for some reason a call_srcu wasn't done. For example, say the hypothetical bug I'm taking about is in tracepoint_remove_func which called the rcu_assign_pointer, but didn't call release_probes:
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c index d0639d917899..f54cb358f451 100644 --- a/kernel/tracepoint.c +++ b/kernel/tracepoint.c @@ -216,7 +216,6 @@ static int tracepoint_add_func(struct tracepoint *tp, rcu_assign_pointer(tp->funcs, tp_funcs); if (!static_key_enabled(&tp->key)) static_key_slow_inc(&tp->key); - release_probes(old); return 0; } --- I want to catching this from the test code. If I did the following, it would be insufficient then: trace_probe_register(...); srcu_barrier(&my_srcu_struct); // only tells me any queued calls // are done but not that something // was queued I was seeing if I could use my_srcu_struct::completed for that. but I'm not sure if that'll work (or that its legal to access it directly - but hey this is just test code! :P).
thanks,
- Joel
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