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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 71EEEC433E0 for ; Thu, 11 Jun 2020 10:37:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 57B65206C3 for ; Thu, 11 Jun 2020 10:37:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726945AbgFKKhM (ORCPT ); Thu, 11 Jun 2020 06:37:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:41902 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726817AbgFKKhL (ORCPT ); Thu, 11 Jun 2020 06:37:11 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 9656BAC46; Thu, 11 Jun 2020 10:37:13 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 70CEE1E1283; Thu, 11 Jun 2020 12:37:09 +0200 (CEST) Date: Thu, 11 Jun 2020 12:37:09 +0200 From: Jan Kara To: Lukas Czerner Cc: linux-ext4@vger.kernel.org, Jan Kara Subject: Re: jbd2: can b_transaction be NULL in refile_buffer ? Message-ID: <20200611103709.GB19132@quack2.suse.cz> References: <20200611083417.4akdykeubd7kfuuh@work> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200611083417.4akdykeubd7kfuuh@work> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org Hi! On Thu 11-06-20 10:34:17, Lukas Czerner wrote: > I am tracking a rare and very hard to reproduce bug that ends up hittng > > J_ASSERT_JH(jh, jh->b_transaction == NULL) > > in __journal_remove_journal_head(). In fact we can get there with > b_next_transaction set and b_jlist == BJ_Forget so it's clear that we > should not have dropped the last JH reference at that point. > > Most of the time that I've seen we get there from > __jbd2_journal_remove_checkpoint() called from > jbd2_journal_commit_transaction(). > > The locking in and around grabbing and putting the journal head > reference (b_jcount) looks solid as well as the use of j_list_lock. But > I have noticed a problem in logic of __jbd2_journal_refile_buffer(). Yeah, the trouble with refcounting bugs is that if *any* of the users releases a reference it should not, we will (much later) hit the problem you describe. > The idea is that b_next_transaction will inherit the reference from > b_transaction so that we do not need to grab a new reference of > journal_head. However this will only be true if b_transaction is set. > > But if it is indeed NULL, then we will do > > WRITE_ONCE(jh->b_transaction, jh->b_next_transaction); > > and __jbd2_journal_file_buffer() will not grab the jh reference. AFAICT > the b_next_transaction is not holding it's own jh reference. This will > result in b_transaction _not_ holding it's own jh reference and we will > be able to drop the last jh reference at unexpected places - hence we can > hit the asserts in __journal_remove_journal_head(). > > However I am not really sure if it is indeed possible to get into > __jbd2_journal_refile_buffer() with b_transaction == NULL and > b_next_transaction set. Jan do you have any idea if that's possible and > what would be the circumstances to lead us there ? __jbd2_journal_refile_buffer() should always be called with b_transaction != NULL and as I've checked (all three) callers, that indeed seems to be the case. Feel free to add assert along those lines to __jbd2_journal_refile_buffer() to see whether it triggers... > Regardless I still think this is a bug in the logic and we should either > make sure that b_transaction is _not_ NULL in > __jbd2_journal_refile_buffer(), or let __jbd2_journal_file_buffer() grab > the jh reference if b_transaction was indeen NULL. How about something > like the following untested patch ? I'd rather got for the assert. It makes things simpler, also the "meaning" of __jbd2_journal_refile_buffer() is "jh is done in its current transaction, deal with it" and that doesn't have a great meaning if b_transaction is NULL. And when you're adding asserts, then adding one in __jbd2_journal_unfile_buffer() checking b_transaction != NULL and b_next_transaction == NULL would be good as well because lot of callers assume this. I've checked the code and I didn't find any problematic one but that code is complex enough that I could have missed something. Honza -- Jan Kara SUSE Labs, CR