From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753685Ab2BBQVS (ORCPT ); Thu, 2 Feb 2012 11:21:18 -0500 Received: from e33.co.us.ibm.com ([32.97.110.151]:33913 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751858Ab2BBQVR (ORCPT ); Thu, 2 Feb 2012 11:21:17 -0500 Date: Thu, 2 Feb 2012 08:20:17 -0800 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org Subject: Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock Message-ID: <20120202162017.GH2518@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120201194131.GA10028@linux.vnet.ibm.com> <1328125319-5205-1-git-send-email-paulmck@linux.vnet.ibm.com> <1328125319-5205-3-git-send-email-paulmck@linux.vnet.ibm.com> <20120202005553.GD29058@leaf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120202005553.GD29058@leaf> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12020216-2398-0000-0000-000003E2996B Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 01, 2012 at 04:55:54PM -0800, Josh Triplett wrote: > On Wed, Feb 01, 2012 at 11:41:21AM -0800, Paul E. McKenney wrote: > > From: "Paul E. McKenney" > > > > It is illegal to have a grace period within a same-flavor RCU read-side > > critical section, so this commit adds lockdep-RCU checks to splat when > > such abuse is encountered. This commit does not detect more elaborate > > RCU deadlock situations. These situations might be a job for lockdep > > enhancements. > > Since doing so also violates the prohibition on blocking within an RCU > read-side critical section, wouldn't it suffice to call might_sleep() or > equivalent, which also detects other problems? (Obviously this doesn't > apply to SRCU, but it applies to the other variants of RCU.) Yes, but... The advantage of the lockdep-RCU splat is that it gives you a better hint as to where the RCU read-side critical section was entered, which is very helpful when tracking these down, especially when they are intermittent. On of the downsides of the Linux kernel community being more RCU-savvy is that the errors they now tend to commit are more complex. ;-) And yes, I should also well check for the other variants of RCU read-side critical section (other than RCU). Done. I also glued the strings together to promote grepability as you suggest later. (But I leave it to you to get checkpatch.pl upgraded -- it currently warns about long lines, but not about strings split across lines.) > > --- a/kernel/rcutiny.c > > +++ b/kernel/rcutiny.c > > @@ -319,6 +319,9 @@ static void rcu_process_callbacks(struct softirq_action *unused) > > */ > > void synchronize_sched(void) > > { > > + rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map), > > + "Illegal grace period in RCU read-side " > > + "critical section"); > > This message doesn't seem entirely obvious to me. A grace period didn't > occur; a synchronize call did, which tried to request a grace period > that can never happen. I suppose I might as well make it consistent with the other messages. ;-) > > --- a/kernel/rcutree.c > > +++ b/kernel/rcutree.c > > @@ -1816,6 +1816,9 @@ EXPORT_SYMBOL_GPL(call_rcu_bh); > > */ > > void synchronize_sched(void) > > { > > + rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map), > > + "Illegal synchronize_sched() in RCU-sched " > > + "read-side critical section"); > > if (rcu_blocking_is_gp()) > > return; > > wait_rcu_gp(call_rcu_sched); > > @@ -1833,6 +1836,9 @@ EXPORT_SYMBOL_GPL(synchronize_sched); > > */ > > void synchronize_rcu_bh(void) > > { > > + rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map), > > + "Illegal synchronize_sched() in RCU-bh " > > + "read-side critical section"); > > Copy-paste problem here: this should say synchronize_sched_bh. (Or > perhaps it should say __func__. :) ) Fixed, but will pass on __func__ for the moment. Cool though it might be to exercise varargs. ;-) > > --- a/kernel/srcu.c > > +++ b/kernel/srcu.c > > @@ -172,6 +172,10 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) > > { > > int idx; > > > > + rcu_lockdep_assert(!lock_is_held(&sp->dep_map), > > + "Illegal SRCU grace period in same-type " > > + "SRCU read-side critical section"); > > Same issue with the message: a grace period didn't occur, and it never > will; a call to synchronize_srcu requesting a grace period occurred. Good catch, fixed! Thanx, Paul