From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BBFF0C04A6B for ; Mon, 6 May 2019 11:24:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 968FB206BF for ; Mon, 6 May 2019 11:24:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726259AbfEFLYu (ORCPT ); Mon, 6 May 2019 07:24:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:35158 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725883AbfEFLYu (ORCPT ); Mon, 6 May 2019 07:24:50 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id E2C16AB6D; Mon, 6 May 2019 11:24:48 +0000 (UTC) Date: Mon, 6 May 2019 13:24:48 +0200 From: Petr Mladek To: Intel Graphics Development , Daniel Vetter , Peter Zijlstra , Ingo Molnar , Will Deacon , Sergey Senozhatsky , Steven Rostedt , John Ogness , linux-kernel@vger.kernel.org Subject: Re: [PATCH] RFC: console: hack up console_lock more v2 Message-ID: <20190506112448.rng2oefmp2c262dq@pathway.suse.cz> References: <20190502141643.21080-1-daniel.vetter@ffwll.ch> <20190506074553.21464-1-daniel.vetter@ffwll.ch> <20190506081614.b7b22k4prodskbiy@pathway.suse.cz> <20190506082628.wehkislebljxmk5d@pathway.suse.cz> <20190506093812.GG17751@phenom.ffwll.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190506093812.GG17751@phenom.ffwll.local> User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 2019-05-06 11:38:13, Daniel Vetter wrote: > On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote: > > On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > > > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > > > console_trylock, called from within printk, can be called from pretty > > > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > > > usually the box is in pretty bad shape at that point already. But it > > > > really doesn't help when then lockdep jumps in and spams the logs, > > > > potentially obscuring the real backtrace we're really interested in. > > > > One case I've seen (slightly simplified backtrace): > > > > > > > > Call Trace: > > > > > > > > console_trylock+0xe/0x60 > > > > vprintk_emit+0xf1/0x320 > > > > printk+0x4d/0x69 > > > > __warn_printk+0x46/0x90 > > > > native_smp_send_reschedule+0x2f/0x40 > > > > check_preempt_curr+0x81/0xa0 > > > > ttwu_do_wakeup+0x14/0x220 > > > > try_to_wake_up+0x218/0x5f0 > > > > > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > > > can get called recursively from printk_safe_up(). > > > > > > And there are more locks taken from try_to_wake_up(), for example, > > > __task_rq_lock() taken from ttwu_remote(). > > > > > > IMHO, the most reliable solution would be do call the entire > > > up_console_sem() from printk deferred context. We could assign > > > few bytes for this context in the per-CPU printk_deferred > > > variable. > > > > Ah, I was too fast and did the same mistake. This won't help because > > it would still call try_to_wake_up() recursively. > > Uh :-/ > > > We need to call all printk's that can be called under locks > > taken in try_to_wake_up() path in printk deferred context. > > Unfortunately it is whack a mole approach. > > Hm since it's whack-a-mole anyway, what about converting the WARN_ON into > a prinkt_deferred, like all the other scheduler related code? Feels a > notch more consistent to me than leaking the printk_context into areas it > wasn't really meant built for. Scheduler code already fully subscribed to > the whack-a-mole approach after all. I am not sure how exactly you mean the conversion. Anyway, we do not want to use printk_deferred() treewide. It reduces the chance that the messages reach consoles. Scheduler is an exception because of the possible deadlocks. A solution would be to define WARN_ON_DEFERRED() that would call normal WARN_ON() in printk deferred context and use in scheduler. Best Regards, Petr