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=-3.0 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 799F4C43219 for ; Fri, 3 May 2019 15:14:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 508E7205ED for ; Fri, 3 May 2019 15:14:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728267AbfECPOk (ORCPT ); Fri, 3 May 2019 11:14:40 -0400 Received: from mx2.suse.de ([195.135.220.15]:51814 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726267AbfECPOj (ORCPT ); Fri, 3 May 2019 11:14:39 -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 71469ACFA; Fri, 3 May 2019 15:14:38 +0000 (UTC) Date: Fri, 3 May 2019 17:14:37 +0200 From: Petr Mladek To: Daniel Vetter Cc: DRI Development , 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_trylock more Message-ID: <20190503151437.dc2ty2mnddabrz4r@pathway.suse.cz> References: <20190502141643.21080-1-daniel.vetter@ffwll.ch> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190502141643.21080-1-daniel.vetter@ffwll.ch> 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 Thu 2019-05-02 16:16:43, 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 > pollwake+0x6f/0x90 > credit_entropy_bits+0x204/0x310 > add_interrupt_randomness+0x18f/0x210 > handle_irq+0x67/0x160 > do_IRQ+0x5e/0x130 > common_interrupt+0xf/0xf > > > This alone isn't a problem, but the spinlock in the semaphore is also > still held while waking up waiters (up() -> __up() -> try_to_wake_up() > callchain), which then closes the runqueue vs. semaphore.lock loop, > and upsets lockdep, which issues a circular locking splat to dmesg. > Worse it upsets developers, since we don't want to spam dmesg with > clutter when the machine is dying already. > > Fix this by creating a __down_trylock which only trylocks the > semaphore.lock. This isn't correct in full generality, but good enough > for console_lock: > > - there's only ever one console_lock holder, we won't fail spuriously > because someone is doing a down() or up() while there's still room > (unlike other semaphores with count > 1). > > - console_unlock() has one massive retry loop, which will catch anyone > who races the trylock against the up(). This makes sure that no > printk lines will get lost. Making the trylock more racy therefore > has no further impact. To be honest, I do not see how this could solve the problem. The circular dependency is still there. If the new __down_trylock() succeeds then console_unlock() will get called in the same context and it will still need to call up() -> try_to_wake_up(). Note that there are many other console_lock() callers that might happen in parallel and might appear in the wait queue. Best Regards, Petr