From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755189Ab2BCJFJ (ORCPT ); Fri, 3 Feb 2012 04:05:09 -0500 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:38682 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755070Ab2BCJFA (ORCPT ); Fri, 3 Feb 2012 04:05:00 -0500 X-Originating-IP: 217.70.178.138 X-Originating-IP: 50.43.15.19 Date: Fri, 3 Feb 2012 01:04:49 -0800 From: Josh Triplett To: "Paul E. McKenney" 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: <20120203090449.GG3008@leaf> 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> <20120202162017.GH2518@linux.vnet.ibm.com> <20120202195638.GA9279@leaf> <20120202204206.GH2518@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120202204206.GH2518@linux.vnet.ibm.com> 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 On Thu, Feb 02, 2012 at 12:42:06PM -0800, Paul E. McKenney wrote: > On Thu, Feb 02, 2012 at 11:56:38AM -0800, Josh Triplett wrote: > > On Thu, Feb 02, 2012 at 08:20:17AM -0800, Paul E. McKenney wrote: > > > 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. > > > > Ah, fair enough. > > > > > And yes, I should also well check for the other variants of RCU read-side > > > critical section (other than RCU). Done. > > > > Oh? What hadn't you checked for? > > Things like synchronize_sched() in rcu_read_lock() critical section > and vice versa. Ouch. Good idea. That also suggests another interesting possibility: lockdep could tag pointers used in the flavor-specific rcu_dereference variants and pointers used in the call_rcu variants to make sure nobody uses multiple variants on the same pointer. :) (Assuming we don't want flavor-specific __rcu_* pointer tags.) Speaking of which, could kfree_rcu require its argument to have the __rcu type annotation? We can't necessarily guarantee that for call_rcu in all cases, but I think we can for kfree_rcu. - Josh Triplett