Re: [PATCH v11 2/3] tracepoint: Make rcuidle tracepoint callers use SRCU

From: Joel Fernandes <joel@joelfernandes.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
	Boqun Feng <boqun.feng@gmail.com>,
	Byungchul Park <byungchul.park@lge.com>,
	Erick Reyes <erickreyes@google.com>,
	Ingo Molnar <mingo@redhat.com>, Julia Cartwright <julia@ni.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Glexiner <tglx@linutronix.de>,
	Todd Kjos <tkjos@google.com>,
	Tom Zanussi <tom.zanussi@linux.intel.com>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v11 2/3] tracepoint: Make rcuidle tracepoint callers use SRCU
Date: Fri, 27 Jul 2018 11:51:13 -0700	[thread overview]
Message-ID: <20180727185113.GA83926@joelaf.mtv.corp.google.com> (raw)
In-Reply-To: <20180727122659.524dd13f@gandalf.local.home>

On Fri, Jul 27, 2018 at 12:26:59PM -0400, Steven Rostedt wrote:
> On Thu, 26 Jul 2018 16:50:43 -0700
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > F
> >  include/linux/tracepoint.h | 41 ++++++++++++++++++++++++++++++--------
> >  kernel/tracepoint.c        | 16 ++++++++++++++-
> >  2 files changed, 48 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index 19a690b559ca..6e7bc6ebfcd8 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -15,6 +15,7 @@
> >   */
> >  
> >  #include <linux/smp.h>
> > +#include <linux/srcu.h>
> >  #include <linux/errno.h>
> >  #include <linux/types.h>
> >  #include <linux/cpumask.h>
> > @@ -33,6 +34,8 @@ struct trace_eval_map {
> >  
> >  #define TRACEPOINT_DEFAULT_PRIO	10
> >  
> > +extern struct srcu_struct tracepoint_srcu;
> > +
> >  extern int
> >  tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
> >  extern int
> > @@ -75,10 +78,16 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
> >   * probe unregistration and the end of module exit to make sure there is no
> >   * caller executing a probe when it is freed.
> >   */
> > +#ifdef CONFIG_TRACEPOINTS
> >  static inline void tracepoint_synchronize_unregister(void)
> >  {
> > +	synchronize_srcu(&tracepoint_srcu);
> >  	synchronize_sched();
> >  }
> > +#else
> > +static inline void tracepoint_synchronize_unregister(void)
> > +{ }
> > +#endif
> >  
> >  #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
> >  extern int syscall_regfunc(void);
> > @@ -129,18 +138,32 @@ extern void syscall_unregfunc(void);
> >   * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
> >   * "void *data", where as the DECLARE_TRACE() will pass in "void *data, proto".
> >   */
> > -#define __DO_TRACE(tp, proto, args, cond, rcucheck)			\
> > +#define __DO_TRACE(tp, proto, args, cond, rcuidle)			\
> >  	do {								\
> >  		struct tracepoint_func *it_func_ptr;			\
> >  		void *it_func;						\
> >  		void *__data;						\
> > +		int __maybe_unused idx = 0;				\
> >  									\
> >  		if (!(cond))						\
> >  			return;						\
> > -		if (rcucheck)						\
> > -			rcu_irq_enter_irqson();				\
> > -		rcu_read_lock_sched_notrace();				\
> > -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\
> > +									\
> > +		/* srcu can't be used from NMI */			\
> > +		if (rcuidle && in_nmi())				\
> > +			WARN_ON_ONCE(1);				\
> 
> Why isn't the above:
> 
> 		WARN_ON_ONCE(rcuidle && in_nmi());
> 
> ?
> 
> The rest looks fine.

When I refactored the code as Peter suggested I happen to move the warning
out but didn't simplify it further (I had it at the back of my head to do
that but missed it).

Since its quite a simple change I can send a follow up patch doing so later,
or can resend this one. What do you suggest?

thanks,

- Joel