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

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Message-ID: <20180712083805.GA67912@joelaf.mtv.corp.google.com>
Date:   Thu, 12 Jul 2018 01:38:05 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.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>,
        kernel-team@...roid.com
Subject: Re: [PATCH v9 5/7] tracing: Centralize preemptirq tracepoints and
 unify their usage

On Wed, Jul 11, 2018 at 09:19:44AM -0400, Steven Rostedt wrote:
> > > 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.

So actually with or without the clean up, I don't see any issues with
dropping lockdep_recursing in my tests at the moment. I'm not sure something
else changed between then and now causing the issue to go away. I can include
Peter's clean up in my series though if he's Ok with it since you guys agree
its a good clean up anyway. Would you prefer I did that, and then also
dropped the lockdep_recursing checks? Or should I keep the
lockdep_recursing() checks just to be safe? Do you see cases where you want
irqsoff tracing while lockdep_recursing() is true?

thanks,

- Joel