From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754871AbaCNPNS (ORCPT ); Fri, 14 Mar 2014 11:13:18 -0400 Received: from cantor2.suse.de ([195.135.220.15]:33961 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753644AbaCNPNR (ORCPT ); Fri, 14 Mar 2014 11:13:17 -0400 Date: Fri, 14 Mar 2014 16:13:14 +0100 From: Jan Kara To: Peter Zijlstra Cc: Jan Kara , Thomas Gleixner , John Stultz , Jiri Bohac , linux-kernel@vger.kernel.org Subject: Re: is printk() safe within a timekeeper_seq write section? Message-ID: <20140314151314.GA5361@quack.suse.cz> References: <20140306174531.GA30634@midget.suse.cz> <531F6429.8050008@linaro.org> <20140312064645.GJ30956@twins.programming.kicks-ass.net> <20140312143456.GB28695@quack.suse.cz> <20140312150617.GE27965@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140312150617.GE27965@twins.programming.kicks-ass.net> 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 Wed 12-03-14 16:06:17, Peter Zijlstra wrote: > On Wed, Mar 12, 2014 at 03:34:56PM +0100, Jan Kara wrote: > > On Wed 12-03-14 07:46:45, Peter Zijlstra wrote: > > > On Tue, Mar 11, 2014 at 10:32:26PM +0100, Thomas Gleixner wrote: > > > > > Peter/Thomas: Any thoughts on the deferred printk buffer? Does printk > > > > > already have something like this? Any other ideas here? > > > > > > > > I was thinking about something like that for RT as on RT printk is a > > > > complete nightmare. It's simple to implement that, but as we know from > > > > the RT experience it can lead to painful loss of debug output. > > > > > > > > Assume you printk inside such a region, which just fills the dmesg > > > > buffer and schedules the delayed output. Now in that same region you > > > > run into a deadlock which causes the whole machine to freeze. Then you > > > > won't see the debug output, which might actually give you the hint why > > > > the system deadlocked .... > > > > > > Ok so I started writing a rant that I don't give a crap about klogd and > > > that deferring that wakeup would be perfectly fine; then I looked at the > > > code and found that we in fact do this already. > > > > > > wake_up_klogd() schedules a lazy irqwork to go wake up, so that's out. > > > > > > That leaves the console sem wakeup; but I suppose we could redo this > > > patch: > > > > > > lkml.kernel.org/r/20110621153806.286257129@chello.nl > > > > > > to get rid of that one. > > I don't know if you've noticed but there's also the following patch: > > https://lkml.org/lkml/2013/12/23/310 > > which would make it pretty easy to just add messages to printk buffer in > > timer code and schedule printing later using irq work. > > Yeah; I suppose that one is prettier. JFYI, that patch has been just added to -mm tree. Honza -- Jan Kara SUSE Labs, CR