From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
"Joel Fernandes, Google" <joel@joelfernandes.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Thomas Gleixner <tglx@linutronix.de>,
paulmck <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook
Date: Tue, 11 Feb 2020 11:35:05 -0500 [thread overview]
Message-ID: <20200211113505.0a1b52e9@gandalf.local.home> (raw)
In-Reply-To: <698566505.617724.1581438456170.JavaMail.zimbra@efficios.com>
On Tue, 11 Feb 2020 11:27:36 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> ----- On Feb 11, 2020, at 11:18 AM, rostedt rostedt@goodmis.org wrote:
>
> > On Tue, 11 Feb 2020 16:34:52 +0100
> > Peter Zijlstra <peterz@infradead.org> wrote:
> >
> >> > + if (unlikely(in_nmi()))
> >> > + goto out;
> >>
> >> unless I'm mistaken, we can simply do rcu_nmi_enter() in this case, and
> >> rcu_nmi_exit() on the other end.
> >>
> >> > + rcu_irq_enter_irqson();
> >
> > The thing is, I don't think this can ever happen. We've had in the
> > tracepoint.h:
> >
> > /* srcu can't be used from NMI */ \
> > WARN_ON_ONCE(rcuidle && in_nmi()); \
> >
> > And this has yet to trigger.
>
> But that "rcuidle" state is defined on a per-tracepoint basis, whereas
> "!rcu_is_watching()" is a state which depends on the current execution
> context. I don't follow how the fact that this WARN_ON_ONCE() never
> triggered allows us to infer anything about (!rcu_is_watching() && in_nmi()).
>
The "_rcuidle()" version of the tracepoint was to be used in places
that RCU may not be watching, otherwise you would get a lockdep splat.
As that "rcuidle" variable is a hardcoded constant, it would be
compiled out when rcuidle is zero. But, in all purposes, rcuidle is
basically equivalent to rcu_is_watching(), because if it wasn't you
would have lockdep splats.
-- Steve
next prev parent reply other threads:[~2020-02-11 16:35 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 14:50 [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook Steven Rostedt
2020-02-11 15:34 ` Mathieu Desnoyers
2020-02-11 15:46 ` Peter Zijlstra
2020-02-11 16:02 ` Mathieu Desnoyers
2020-02-11 15:34 ` Peter Zijlstra
2020-02-11 16:18 ` Steven Rostedt
2020-02-11 16:27 ` Mathieu Desnoyers
2020-02-11 16:35 ` Steven Rostedt [this message]
2020-02-11 17:29 ` Peter Zijlstra
2020-02-11 17:32 ` Peter Zijlstra
2020-02-11 18:54 ` Paul E. McKenney
2020-02-12 8:05 ` Peter Zijlstra
2020-02-12 9:05 ` Paul E. McKenney
2020-02-11 17:35 ` Mathieu Desnoyers
2020-02-12 8:02 ` Peter Zijlstra
2020-02-12 15:14 ` Mathieu Desnoyers
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