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Date: Wed, 11 Jul 2018 21:26:26 -0400 From: Steven Rostedt <rostedt@...dmis.org> To: Joel Fernandes <joel@...lfernandes.org> Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>, Byungchul Park <byungchul.park@....com>, Ingo Molnar <mingo@...hat.com>, Julia Cartwright <julia@...com>, linux-kselftest@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Namhyung Kim <namhyung@...nel.org>, Paul McKenney <paulmck@...ux.vnet.ibm.com>, Thomas Glexiner <tglx@...utronix.de>, Tom Zanussi <tom.zanussi@...ux.intel.com> Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU On Wed, 11 Jul 2018 17:31:00 -0700 Joel Fernandes <joel@...lfernandes.org> wrote: > On Wed, Jul 11, 2018 at 09:06:49AM -0400, Steven Rostedt wrote: > > On Wed, 11 Jul 2018 14:56:47 +0200 > > Peter Zijlstra <peterz@...radead.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. > > But isn't the point of synchronize_rcu to make sure that we're no longer in > an RCU read-side section, not that *all* queued callbacks already ran? So in that > case, I think it doesn't matter which order the 2 synchronize functions are > called in. Please let me know if if I missed something! > > I believe what we're trying to guarantee here is that no tracepoints using > either flavor of RCU are active after tracepoint_synchronize_unregister > returns. Yes you are correct. If tracepoint_synchronize_unregister() is only to make sure that there is no more trace events using the probes, then this should work. I was focused on looking at it with release_probes() too. So the patch is fine as is. -- Steve
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