linux-kernel - Re: [PATCH v9 5/7] tracing: Centralize preemptirq tracepoints and unify their usage

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Date:   Wed, 11 Jul 2018 09:19:44 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Joel Fernandes <joel@...lfernandes.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 5/7] tracing: Centralize preemptirq tracepoints and
 unify their usage

On Wed, 11 Jul 2018 15:12:56 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jun 28, 2018 at 11:21:47AM -0700, Joel Fernandes wrote:
> > One note, I have to check for lockdep recursion in the code that calls
> > the trace events API and bail out if we're in lockdep recursion  
> 
> I'm not seeing any new lockdep_recursion checks...

I believe he's talking about this part:

+void trace_hardirqs_on(void)
+{
+	if (lockdep_recursing(current) || !this_cpu_read(tracing_irq_cpu))
+		return;
+

[etc]

> 
> > protection to prevent something like the following case: a spin_lock is
> > taken. Then lockdep_acquired is called.  That does a raw_local_irq_save
> > and then sets lockdep_recursion, and then calls __lockdep_acquired. In
> > this function, a call to get_lock_stats happens which calls
> > preempt_disable, which calls trace IRQS off somewhere which enters my
> > tracepoint code and sets the tracing_irq_cpu flag to prevent recursion.
> > This flag is then never cleared causing lockdep paths to never be
> > entered and thus causing splats and other bad things.  
> 
> Would it not be much easier to avoid that entirely, afaict all
> get/put_lock_stats() callers already have IRQs disabled, so that
> (traced) preempt fiddling is entirely superfluous.

Agreed. Looks like a good clean up.

-- Steve