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

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Date:   Wed, 11 Jul 2018 17:44:11 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        Steven Rostedt <rostedt@...dmis.org>,
        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, Jul 11, 2018 at 03:12:56PM +0200, Peter Zijlstra 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 meant its this part:

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

> > 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.

Let me try to apply Peter's diff and see if I still don't need lockdep
recursion checking. I believe it is still harmless to still check for lockdep
recursion just to be safe. But I'll give it a try and let you know how it
goes.

thanks!

- Joel


> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 5fa4d3138bf1..8f5ce0048d15 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -248,12 +248,7 @@ void clear_lock_stats(struct lock_class *class)
>  
>  static struct lock_class_stats *get_lock_stats(struct lock_class *class)
>  {
> -	return &get_cpu_var(cpu_lock_stats)[class - lock_classes];
> -}
> -
> -static void put_lock_stats(struct lock_class_stats *stats)
> -{
> -	put_cpu_var(cpu_lock_stats);
> +	return &this_cpu_ptr(&cpu_lock_stats)[class - lock_classes];
>  }
>  
>  static void lock_release_holdtime(struct held_lock *hlock)
> @@ -271,7 +266,6 @@ static void lock_release_holdtime(struct held_lock *hlock)
>  		lock_time_inc(&stats->read_holdtime, holdtime);
>  	else
>  		lock_time_inc(&stats->write_holdtime, holdtime);
> -	put_lock_stats(stats);
>  }
>  #else
>  static inline void lock_release_holdtime(struct held_lock *hlock)
> @@ -4090,7 +4084,6 @@ __lock_contended(struct lockdep_map *lock, unsigned long ip)
>  		stats->contending_point[contending_point]++;
>  	if (lock->cpu != smp_processor_id())
>  		stats->bounces[bounce_contended + !!hlock->read]++;
> -	put_lock_stats(stats);
>  }
>  
>  static void
> @@ -4138,7 +4131,6 @@ __lock_acquired(struct lockdep_map *lock, unsigned long ip)
>  	}
>  	if (lock->cpu != cpu)
>  		stats->bounces[bounce_acquired + !!hlock->read]++;
> -	put_lock_stats(stats);
>  
>  	lock->cpu = cpu;
>