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In-Reply-To: <20180504234342.GJ26088@linux.vnet.ibm.com>
From:   Joel Fernandes <joelaf@google.com>
Date:   Sat, 05 May 2018 00:39:36 +0000
Message-ID: <CAJWu+ornetogyZ4orGFsea-urbdJC2+3UhCT_FKVmJ_EHncX1A@mail.gmail.com>
Subject: Re: rcu-bh design
To:     Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc:     "Joel Fernandes (Google)" <joel.opensrc@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, May 4, 2018 at 4:42 PM Paul E. McKenney <paulmck@linux.vnet.ibm.com>
wrote:
[..]
> > 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).

> This is a bit ugly to do programmatically in the kernel.  The callbacks
> are on per-CPU queues that cannot be safely accessed except by that CPU.
> Plus they are on singly linked lists that can be quite long, so it could
> be insanely slow as well.

> But given that this is test code, why not leverage event tracing?
> There are some relevant events in include/trace/events/rcu.h, starting
> with the rcu_callback trace event.


Sounds like a great idea. I'll try that, thanks.

- Joel