From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC] v7 expedited "big hammer" RCU grace periods Date: Mon, 25 May 2009 18:28:43 -0700 Message-ID: <20090526012843.GF7168@linux.vnet.ibm.com> References: <20090522190525.GA13286@linux.vnet.ibm.com> <4A1A3C23.8090004@cn.fujitsu.com> <20090525164446.GD7168@linux.vnet.ibm.com> <4A1B3FFB.7090306@cn.fujitsu.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, mingo@elte.hu, akpm@linux-foundation.org, torvalds@linux-foundation.org, davem@davemloft.net, dada1@cosmosbay.com, zbr@ioremap.net, jeff.chua.linux@gmail.com, paulus@samba.org, jengelh@medozas.de, r000n@r000n.net, benh@kernel.crashing.org, mathieu.desnoyers@polymtl.ca To: Lai Jiangshan Return-path: Content-Disposition: inline In-Reply-To: <4A1B3FFB.7090306@cn.fujitsu.com> Sender: netfilter-devel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, May 26, 2009 at 09:03:55AM +0800, Lai Jiangshan wrote: > Paul E. McKenney wrote: > > > > Good point -- I should at the very least add a comment to > > synchronize_sched_expedited() stating that it cannot be called holding > > any lock that is acquired in a CPU hotplug notifier. If this restriction > > causes any problems, then your approach seems like a promising fix. > > Reviewed-by: Lai Jiangshan Thank you very much for your review and comments!!! > >> The coupling of synchronize_sched_expedited() and migration_req > >> is largely increased: > >> > >> 1) The offline cpu's per_cpu(rcu_migration_req, cpu) is handled. > >> See migration_call::CPU_DEAD > > > > Good. ;-) > > > >> 2) migration_call() is the highest priority of cpu notifiers, > >> So even any other cpu notifier calls synchronize_sched_expedited(), > >> It'll not cause DEADLOCK. > > > > You mean if using your preempt_disable() approach, right? Unless I am > > missing something, the current get_online_cpus() approach would deadlock > > in this case. > > Yes, I mean if using my preempt_disable() approach. The current > get_online_cpus() approach would NOT deadlock in this case also, > we can require get_online_cpus() in cpu notifiers. I have added the comment for the time being, but should people need to use this in CPU-hotplug notifiers, then again your preempt_disable() approach looks to be a promising fix. > > diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h > > index fce5227..78117ed 100644 > > --- a/include/linux/rcupreempt.h > > +++ b/include/linux/rcupreempt.h > > @@ -74,6 +74,16 @@ extern int rcu_needs_cpu(int cpu); > > > > extern void __synchronize_sched(void); > > > > +static inline void synchronize_rcu_expedited(void) > > +{ > > + synchronize_rcu(); /* Placeholder for new rcupreempt implementation. */ > > +} > > + > > +static inline void synchronize_rcu_bh_expedited(void) > > +{ > > + synchronize_rcu(); /* Placeholder for new rcupreempt implementation. */ > > +} > > + > > Why not synchronize_rcu_bh() ? Ummm... Because I did a typo. Fixed. > In mainline, rcu_read_lock_bh() is not preemptable, > So I think synchronize_sched_expedited() is better. > > Anyway, synchronize_rcu() is OK for me, because it is > "Placeholder for new rcupreempt implementation". And I am worried that preemptable RCU's rcu_bh read sides might someday become preemptable. Seems a bit unlikely at this point, but why tempt fate? Thanx, Paul