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

From: Steven Rostedt <rostedt@goodmis.org>
To: Joel Fernandes <joel@joelfernandes.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 12:26:59 -0400	[thread overview]
Message-ID: <20180727122659.524dd13f@gandalf.local.home> (raw)
In-Reply-To: <20180726235044.10195-3-joel@joelfernandes.org>

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.

-- Steve

> +									\
> +		/* keep srcu and sched-rcu usage consistent */		\
> +		preempt_disable_notrace();				\
> +									\
> +		/*							\
> +		 * For rcuidle callers, use srcu since sched-rcu	\
> +		 * doesn't work from the idle path.			\
> +		 */							\
> +		if (rcuidle)						\
> +			idx = srcu_read_lock_notrace(&tracepoint_srcu);	\
> +									\
> +		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
> +									\
>  		if (it_func_ptr) {					\
>  			do {						\
>  				it_func = (it_func_ptr)->func;		\
> @@ -148,9 +171,11 @@ extern void syscall_unregfunc(void);
>  				((void(*)(proto))(it_func))(args);	\
>  			} while ((++it_func_ptr)->func);		\
>  		}							\
> -		rcu_read_unlock_sched_notrace();			\
> -		if (rcucheck)						\
> -			rcu_irq_exit_irqson();				\
> +									\
> +		if (rcuidle)						\
> +			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
> +									\
> +		preempt_enable_notrace();				\
>  	} while (0)
>  
> 