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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT 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 C5FA8C4360F for ; Wed, 3 Apr 2019 17:33:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 886DC206DF for ; Wed, 3 Apr 2019 17:33:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726511AbfDCRdI (ORCPT ); Wed, 3 Apr 2019 13:33:08 -0400 Received: from foss.arm.com ([217.140.101.70]:45670 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726084AbfDCRdI (ORCPT ); Wed, 3 Apr 2019 13:33:08 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D04B980D; Wed, 3 Apr 2019 10:33:07 -0700 (PDT) Received: from fuggles.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 798193F68F; Wed, 3 Apr 2019 10:33:06 -0700 (PDT) Date: Wed, 3 Apr 2019 18:33:04 +0100 From: Will Deacon To: Bart Van Assche Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, Waiman Long , shenghui Subject: Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled Message-ID: <20190403173304.GI17500@fuggles.cambridge.arm.com> References: <20190326195912.43316-1-bvanassche@acm.org> <20190403124410.GE16362@fuggles.cambridge.arm.com> <1554306482.118779.198.camel@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1554306482.118779.198.camel@acm.org> User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 03, 2019 at 08:48:02AM -0700, Bart Van Assche wrote: > On Wed, 2019-04-03 at 13:44 +0100, Will Deacon wrote: > > On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote: > > > Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer > > > in use") changed the behavior of lockdep_free_key_range() from > > > unconditionally zapping lock classes into only zapping lock classes if > > > debug_lock == true. Since the new behavior can cause cat /proc/lockdep to > > > crash due to a NULL pointer dereference, restore the pre-v5.1 behavior. > > > > Can you elaborate on this NULL dereference please, and why this patch fixes > > it? > > Not zapping lock classes if debug_lock == false leaves dangling pointers in > several lockdep datastructures, e.g. lock_class::name in the all_lock_classes > list. The shell command "cat /proc/lockdep" causes the kernel to iterate the > all_lock_classes list. Hence the "unable to handle kernel paging request" > issue that Shenghui encountered by running cat /proc/lockdep. Please let me > know if you would like me to repost this patch with a more detailed > description. That would help me, at least (maybe with the crash log). Thanks. > > > Cc: Thomas Gleixner > > > Cc: Will Deacon > > > Cc: Waiman Long > > > Cc: shenghui > > > Reported-by: shenghui > > > Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1. > > > Signed-off-by: Bart Van Assche > > > --- > > > kernel/locking/lockdep.c | 23 ++++++----------------- > > > 1 file changed, 6 insertions(+), 17 deletions(-) > > > > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > > > index 34cdcbedda49..70480e4f8f5d 100644 > > > --- a/kernel/locking/lockdep.c > > > +++ b/kernel/locking/lockdep.c > > > @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch) > > > return; > > > > > > raw_local_irq_save(flags); > > > - if (!graph_lock()) > > > - goto out_irq; > > > + arch_spin_lock(&lockdep_lock); > > > > This also throws out the recursion counting. Is that ok? > > I think that that's OK. My understanding is that lockdep keeps track of > recursion to avoid that lockdep_lock is locked recursively. However, none > of the functions modified by this patch are called with that lock held. Might be worth adding a comment to that effect, so people don't change that in future. Will