From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934299AbdDGQow (ORCPT ); Fri, 7 Apr 2017 12:44:52 -0400 Received: from mail.kernel.org ([198.145.29.136]:37246 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934092AbdDGQop (ORCPT ); Fri, 7 Apr 2017 12:44:45 -0400 Date: Fri, 7 Apr 2017 12:44:40 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton Subject: Re: [PATCH 6/5]rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work Message-ID: <20170407124440.7697e14f@gandalf.local.home> In-Reply-To: <20170407164220.GY1600@linux.vnet.ibm.com> References: <20170407140106.051135969@goodmis.org> <20170407123516.0560f48c@gandalf.local.home> <20170407164220.GY1600@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 7 Apr 2017 09:42:20 -0700 "Paul E. McKenney" wrote: > On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote: > > From: "Steven Rostedt (VMware)" > > > > Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when > > it needs to use rcu_read_lock() and friends. This is because tracing can > > happen as RCU is about to enter user space, or about to go idle, and RCU > > does not watch for RCU read side critical sections as it makes the > > transition. > > > > There is a small location within the RCU infrastructure that rcu_irq_enter() > > itself will not work. If tracing were to occur in that section it will break > > if it tries to use rcu_irq_enter(). > > > > Originally, this happens with the stack_tracer, because it will call > > save_stack_trace when it encounters stack usage that is greater than any > > stack usage it had encountered previously. There was a case where that > > happened in the RCU section where rcu_irq_enter() did not work, and lockdep > > complained loudly about it. To fix it, stack tracing added a call to be > > disabled and RCU would disable stack tracing during the critical section > > that rcu_irq_enter() was inoperable. This solution worked, but there are > > other cases that use rcu_irq_enter() and it would be a good idea to let RCU > > give a way to let others know that rcu_irq_enter() will not work. For > > example, in trace events. > > > > Another helpful aspect of this change is that it also moves the per cpu > > variable called in the RCU critical section into a cache locale along with > > other RCU per cpu variables used in that same location. > > > > I'm keeping the stack_trace_disable() code, as that still could be used in > > the future by places that really need to disable it. And since it's only a > > static inline, it wont take up any kernel text if it is not used. > > > > Signed-off-by: Steven Rostedt (VMware) > > Looks better, especially __this_cpu_inc() and __this_cpu_dec(). > > How about rcu_irq_enter_disabled instead of rcu_disabled? We aren't > really disabling all of RCU. ;-) OK, I'll make the update and send a 6.1 (and also a new 7/5)! -- Steve