Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758701Ab3IFQk0 (ORCPT ); Fri, 6 Sep 2013 12:40:26 -0400 Received: from mail-we0-f181.google.com ([74.125.82.181]:40531 "EHLO mail-we0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758030Ab3IFQkW (ORCPT ); Fri, 6 Sep 2013 12:40:22 -0400 Date: Fri, 6 Sep 2013 18:40:18 +0200 From: Frederic Weisbecker To: Steven Rostedt Cc: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, sbw@mit.edu Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section? Message-ID: <20130906164016.GB2706@somewhere> References: <20130905195234.GA20555@linux.vnet.ibm.com> <20130906105934.GF20519@somewhere> <20130906151851.GQ3966@linux.vnet.ibm.com> <20130906113320.46b2ea3e@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130906113320.46b2ea3e@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3875 Lines: 99 On Fri, Sep 06, 2013 at 11:33:20AM -0400, Steven Rostedt wrote: > On Fri, 6 Sep 2013 08:18:52 -0700 > "Paul E. McKenney" wrote: > > > On Fri, Sep 06, 2013 at 12:59:41PM +0200, Frederic Weisbecker wrote: > > > On Thu, Sep 05, 2013 at 12:52:34PM -0700, Paul E. McKenney wrote: > > > > There is currently no way for kernel code to determine whether it > > > > is safe to enter an RCU read-side critical section, in other words, > > > > whether or not RCU is paying attention to the currently running CPU. > > > > Given the large and increasing quantity of code shared by the idle loop > > > > and non-idle code, the this shortcoming is becoming increasingly painful. > > > > > > > > This commit therefore adds rcu_watching_this_cpu(), which returns true > > > > if it is safe to enter an RCU read-side critical section on the currently > > > > running CPU. This function is quite fast, using only a __this_cpu_read(). > > > > However, the caller must disable preemption. > > > > > > > > Reported-by: Steven Rostedt > > > > Signed-off-by: Paul E. McKenney > > > > > > > > include/linux/rcupdate.h | 1 + > > > > kernel/rcutree.c | 12 ++++++++++++ > > > > 2 files changed, 13 insertions(+) > > > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > > index 15d33d9..1c7112c 100644 > > > > --- a/include/linux/rcupdate.h > > > > +++ b/include/linux/rcupdate.h > > > > @@ -225,6 +225,7 @@ extern void rcu_idle_enter(void); > > > > extern void rcu_idle_exit(void); > > > > extern void rcu_irq_enter(void); > > > > extern void rcu_irq_exit(void); > > > > +extern bool rcu_watching_this_cpu(void); > > > > > > > > #ifdef CONFIG_RCU_USER_QS > > > > extern void rcu_user_enter(void); > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > index a06d172..7b8fcee 100644 > > > > --- a/kernel/rcutree.c > > > > +++ b/kernel/rcutree.c > > > > @@ -710,6 +710,18 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online); > > > > #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */ > > > > > > > > /** > > > > + * rcu_watching_this_cpu - are RCU read-side critical sections safe? > > > > + * > > > > + * Return true if RCU is watching the running CPU, which means that this > > > > + * CPU can safely enter RCU read-side critical sections. The caller must > > > > + * have at least disabled preemption. > > > > + */ > > > > +bool rcu_watching_this_cpu(void) > > > > +{ > > > > + return !!__this_cpu_read(rcu_dynticks.dynticks_nesting); > > > > +} > > > > > > There is also rcu_is_cpu_idle(). > > > > Good point, thank you! I was clearly in autonomic-reflex mode yesterday. :-/ > > > > Here is the rcutree version: > > > > int rcu_is_cpu_idle(void) > > { > > int ret; > > > > preempt_disable(); > > ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; > > preempt_enable(); > > return ret; > > } > > > > And here is the rcutiny version: > > > > int rcu_is_cpu_idle(void) > > { > > return !rcu_dynticks_nesting; > > } > > > > Steve, could you please use rcu_is_cpu_idle()? I will revert yesterday's > > redundancy. > > > > I can't use plain preempt_disable() in function tracing. > > Also, since it's a misnomer to say the cpu is idle in NO_HZ_FULL when > we are coming from userspace, can we rename that? > > Perhaps we can also have a __rcu_is_cpu_tracking() (or whatever), with > the "__" appended that does not do the preempt disable. rcu_is_cpu_eqs() is probably better. It refers to other related "eqs" naming in RCU APIs. > -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/