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=-0.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 66C67C65BAF for ; Wed, 12 Dec 2018 23:39:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2C6A920851 for ; Wed, 12 Dec 2018 23:39:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2C6A920851 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728058AbeLLXjG (ORCPT ); Wed, 12 Dec 2018 18:39:06 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:41746 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726214AbeLLXjF (ORCPT ); Wed, 12 Dec 2018 18:39:05 -0500 Received: from localhost.localdomain (c-24-6-170-16.hsd1.ca.comcast.net [24.6.170.16]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id A0A50F1E; Wed, 12 Dec 2018 23:39:03 +0000 (UTC) Date: Wed, 12 Dec 2018 15:39:01 -0800 From: Andrew Morton To: Waiman Long Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Yang Shi , Arnd Bergmann , Sergey Senozhatsky , Dmitry Safonov Subject: Re: [PATCH] debugobjects: Move printk out of db lock critical sections Message-Id: <20181212153901.4e9c142cf2d6a75373d28368@linux-foundation.org> In-Reply-To: <1544653694-27873-1-git-send-email-longman@redhat.com> References: <1544653694-27873-1-git-send-email-longman@redhat.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 12 Dec 2018 17:28:14 -0500 Waiman Long wrote: > The db->lock is a raw spinlock and so the lock hold time is supposed > to be short. This will not be the case when printk() is being involved > in some of the critical sections. In order to avoid the long hold time, > in case some messages need to be printed, the debug_object_is_on_stack() > and debug_print_object() calls are now moved out of those critical > sections. > > Holding the db->lock while calling printk() may lead to deadlock if > printk() somehow requires the allocation/freeing of debug object that > happens to be in the same hash bucket or a circular lock dependency > warning from lockdep as reported in https://lkml.org/lkml/2018/12/11/143. > > [ 87.209665] WARNING: possible circular locking dependency detected > [ 87.210547] 4.20.0-rc4-00057-gc96cf92 #1 Tainted: G W > [ 87.211449] ------------------------------------------------------ > [ 87.212405] getty/519 is trying to acquire lock: > [ 87.213074] (____ptrval____) (&obj_hash[i].lock){-.-.}, at: debug_check_no_obj_freed+0xb4/0x302 > [ 87.214343] > [ 87.214343] but task is already holding lock: > [ 87.215174] (____ptrval____) (&port_lock_key){-.-.}, at: uart_shutdown+0x3a3/0x4e2 > [ 87.216260] > [ 87.216260] which lock already depends on the new lock. > > This patch was also found to be able to fix a boot hanging problem > when the initramfs image was switched on after a debugobjects splat > from the EFI code. Patch looks sensible, but I have a nit about the variable names. > --- a/lib/debugobjects.c > +++ b/lib/debugobjects.c > @@ -375,6 +375,8 @@ static void debug_object_is_on_stack(void *addr, int onstack) > struct debug_bucket *db; > struct debug_obj *obj; > unsigned long flags; > + bool debug_printobj = false; "debug_printobject" would be better, but this code already intermingles "obj" and "object". > + bool debug_chkstack = false; Not so good. Is it debug_chkstack or debug_checkstk or ... This file uses "check" consistently so let's not depart from that? Linux style is to avoid these tricky little abbreviations and to use full words. ie, debug_checkstack, please. Better would be debug_check_stack. Or simply check_stack: the "debug" doesn't add anything useful.