From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932065AbdDDPQT (ORCPT ); Tue, 4 Apr 2017 11:16:19 -0400 Received: from mail-pg0-f67.google.com ([74.125.83.67]:35300 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752659AbdDDPQR (ORCPT ); Tue, 4 Apr 2017 11:16:17 -0400 Date: Wed, 5 Apr 2017 01:15:01 +0900 From: Sergey Senozhatsky To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Jan Kara , Andrew Morton , Linus Torvalds , Peter Zijlstra , "Rafael J . Wysocki" , Eric Biederman , Greg Kroah-Hartman , Jiri Slaby , Pavel Machek , Len Brown , linux-kernel@vger.kernel.org, Sergey Senozhatsky Subject: Re: [RFC][PATCHv2 3/8] printk: offload printing from wake_up_klogd_work_func() Message-ID: <20170404161501.GA498@tigerII.localdomain> References: <20170329092511.3958-1-sergey.senozhatsky@gmail.com> <20170329092511.3958-4-sergey.senozhatsky@gmail.com> <20170331145602.GB3452@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170331145602.GB3452@pathway.suse.cz> User-Agent: Mutt/1.8.0 (2017-02-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Petr, sorry for the delay. On (03/31/17 16:56), Petr Mladek wrote: [..] > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > > index ab6b3b2a68c6..1927b5cb5cbe 100644 > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -2741,8 +2741,16 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work) > > * If trylock fails, someone else is doing the printing. > > * PRINTK_PENDING_OUTPUT bit is cleared by console_unlock(). > > */ > > - if (console_trylock()) > > - console_unlock(); > > + if (printk_kthread_enabled()) { > > + wake_up_process(printk_kthread); > > Note that the relation between printk_kthread_enabled() > and wake_up_process() is racy. The conditions might change > between these two calls. It looks fine here, well almost. > > The critical point is in vprintk_emit(). It must use the emergency > mode (call the consoles directly) when it is called from a process > that started the emergency mode. hm, we don't guarantee this. printk(), both in threaded and in emergency modes, can fail to acquire console_sem. > We could be more relaxed here. IMHO, the only sensitive situation > is if printk_deferred() is used in the emergency context. > We might want to use the emergency mode here as well but > it is not guaranteed. hm, I don't think any path does printk_emergency_begin() printk_deferred() printk_emergency_end() and expects logbuf output to be flushed by the time it does printk_emergency_end(). it's most likely something like this printk_emergency_begin() printk() printk_emergency_end() the expectations here are more reasonable, but still, no guarantees are provided (even in non-kthreaded printk mode). > A solution might be to add one more bit, e.g. > PRINTK_PENDING_EMERGENCY_OUTPUT. We should force the emergency mode > here when it is set. It should be cleared together with the normal > PRINTK_PENDING_OUTPUT. > > Or do you think that this is a corner case that we could > ignore for now? hm, I guess we don't really count on irq_work in emergency situations. but I need more time to think. good questions, Petr. -ss