From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753818Ab2E2Lzb (ORCPT ); Tue, 29 May 2012 07:55:31 -0400 Received: from e28smtp07.in.ibm.com ([122.248.162.7]:57561 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752768Ab2E2Lz3 (ORCPT ); Tue, 29 May 2012 07:55:29 -0400 Message-ID: <4FC4B902.7030702@linux.vnet.ibm.com> Date: Tue, 29 May 2012 17:24:42 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120424 Thunderbird/12.0 MIME-Version: 1.0 To: Steven Rostedt CC: "Rafael J. Wysocki" , mingo@kernel.org, lenb@kernel.org, pavel@ucw.cz, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] ftrace: Disable function tracing during suspend/resume and hibernation, again References: <20120521065651.4142.36898.stgit@srivatsabhat.in.ibm.com> <201205212355.45929.rjw@sisk.pl> <1337638586.13348.7.camel@gandalf.stny.rr.com> <4FBB2E38.60804@linux.vnet.ibm.com> In-Reply-To: <4FBB2E38.60804@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit x-cbid: 12052911-8878-0000-0000-000002B446C2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/22/2012 11:42 AM, Srivatsa S. Bhat wrote: > On 05/22/2012 03:46 AM, Steven Rostedt wrote: > >> On Mon, 2012-05-21 at 23:55 +0200, Rafael J. Wysocki wrote: >>> On Monday, May 21, 2012, Srivatsa S. Bhat wrote: >>>> If function tracing is enabled for some of the low-level suspend/resume >>>> functions, it leads to triple fault during resume from suspend, ultimately >>>> ending up in a reboot instead of a resume (or a total refusal to come out >>>> of suspended state, on some machines). >>>> >>>> This issue was explained in more detail in commit f42ac38c59e0a03d (ftrace: >>>> disable tracing for suspend to ram). However, the changes made by that commit >>>> got reverted by commit cbe2f5a6e84eebb (tracing: allow tracing of >>>> suspend/resume & hibernation code again). So, unfortunately since things are >>>> not yet robust enough to allow tracing of low-level suspend/resume functions, >>>> suspend/resume is still broken when ftrace is enabled. >>>> >>>> So fix this by disabling function tracing during suspend/resume & hibernation. >>> >>> Steve, any comments? >> >> I was actually the one to recommend this solution ;-) >> > > > And I am grateful to you for that :-) > >> But that said, I actually do have some. >> >>> >>>> Signed-off-by: Srivatsa S. Bhat >>>> Cc: stable@vger.kernel.org >>>> --- >>>> >>>> kernel/power/hibernate.c | 6 ++++++ >>>> kernel/power/suspend.c | 3 +++ >>>> 2 files changed, 9 insertions(+), 0 deletions(-) >>>> >>>> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c >>>> index e09dfbf..52a1817 100644 >>>> --- a/kernel/power/hibernate.c >>>> +++ b/kernel/power/hibernate.c >>>> @@ -352,6 +352,7 @@ int hibernation_snapshot(int platform_mode) >>>> } >>>> >>>> suspend_console(); >>>> + ftrace_stop(); >> >> >> Is this a point of no return? >> > > Yes, especially as far as userspace is concerned. > >> This probably isn't a problem, but I'm being a little paranoid. As >> ftrace_stop/start() is just an on off switch, anyplace that calls >> ftrace_start() will enable function tracing again. Ftrace_start() is >> called by tracing_start() which is called when >> the /sys/kernel/debug/tracing/trace file is opened and then closed. If >> the close happens while the suspend is starting, it possibly can start >> function tracing again. >> >> But this race would have to be done by the user, which should have full >> control as they are in the process of suspending the machine. But they >> may do it by accident. >> > > > Luckily, at all these places, all the userspace processes (and the freezable > kernel threads) are already frozen. So nobody will be running at this point > to open/close the trace file. > > So there is no problem right? > > (Of course, the non-freezable kernel threads might be running at this point, > but IIUC, they can't trigger ftrace_start() from some other internal path..) > Steve, do you have any other concerns about this patch or is it fine? (I addressed your concern about some other event leading to ftrace_start() above). Regards, Srivatsa S. Bhat