Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752993Ab3FDLDk (ORCPT ); Tue, 4 Jun 2013 07:03:40 -0400 Received: from mail-ea0-f179.google.com ([209.85.215.179]:57274 "EHLO mail-ea0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751793Ab3FDLDf (ORCPT ); Tue, 4 Jun 2013 07:03:35 -0400 Date: Tue, 4 Jun 2013 13:03:31 +0200 From: Frederic Weisbecker To: Steven Rostedt Cc: "Paul E. McKenney" , LKML , Tejun Heo , Ingo Molnar , Peter Zijlstra , Jiri Olsa Subject: Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize_sched() Message-ID: <20130604110329.GA14973@somewhere> References: <1369785676.15552.55.camel@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1369785676.15552.55.camel@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: 2158 Lines: 45 On Tue, May 28, 2013 at 08:01:16PM -0400, Steven Rostedt wrote: > The function tracer uses preempt_disable/enable_notrace() for > synchronization between reading registered ftrace_ops and unregistering > them. > > Most of the ftrace_ops are global permanent structures that do not > require this synchronization. That is, ops may be added and removed from > the hlist but are never freed, and wont hurt if a synchronization is > missed. > > But this is not true for dynamically created ftrace_ops or control_ops, > which are used by the perf function tracing. > > The problem here is that the function tracer can be used to trace > kernel/user context switches as well as going to and from idle. > Basically, it can be used to trace blind spots of the RCU subsystem. > This means that even though preempt_disable() is done, a > synchronize_sched() will ignore CPUs that haven't made it out of user > space or idle. These can include functions that are being traced just > before entering or exiting the kernel sections. > > To implement the RCU synchronization, instead of using > synchronize_sched() the use of schedule_on_each_cpu() is performed. This > means that when a dynamically allocated ftrace_ops, or a control ops is > being unregistered, all CPUs must be touched and execute a ftrace_sync() > stub function via the work queues. This will rip CPUs out from idle or > in dynamic tick mode. This only happens when a user disables perf > function tracing or other dynamically allocated function tracers, but it > allows us to continue to debug RCU and context tracking with function > tracing. > > Signed-off-by: Steven Rostedt Acked-by: Frederic Weisbecker If ftrace were to use rcu_dereference_sched() instead of rcu_dereference_raw(), I guess the issue would have been detected before. But I guess we want to avoid that for tracing recursion? Thanks. -- 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/