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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 11FCDC433E1 for ; Thu, 11 Jun 2020 10:50:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E25D7207C3 for ; Thu, 11 Jun 2020 10:50:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZdErC08X" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727110AbgFKKuh (ORCPT ); Thu, 11 Jun 2020 06:50:37 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:20543 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727097AbgFKKud (ORCPT ); Thu, 11 Jun 2020 06:50:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591872631; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=kSdiDpqI3jPoATNC/+3bJgMnBdq/VMgYcXdBesTmLZU=; b=ZdErC08X1/QiepzfTVEfh6DBBoq8+wNf3LkIbKScQZlYSvJ+oJRV+2ic+t1t5+a2y3HMc2 q2sO+r/WV/0iYe1IXHysK5PWzwnQr6gK0E7iZjIxeRBdlbPWGtoqZPI9n7AGpvMgl6so0a cUsXbM6JZOAJCh/0SSP7GcmvICG1JvA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-121-WQDdQtVxOnioec_cgbm-bg-1; Thu, 11 Jun 2020 06:50:29 -0400 X-MC-Unique: WQDdQtVxOnioec_cgbm-bg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B3F4819200C0; Thu, 11 Jun 2020 10:50:28 +0000 (UTC) Received: from work (unknown [10.40.192.76]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 02CA27F4E5; Thu, 11 Jun 2020 10:50:27 +0000 (UTC) Date: Thu, 11 Jun 2020 12:50:24 +0200 From: Lukas Czerner To: Jan Kara Cc: linux-ext4@vger.kernel.org Subject: Re: jbd2: can b_transaction be NULL in refile_buffer ? Message-ID: <20200611105024.tjykjnjkswsar3ah@work> References: <20200611083417.4akdykeubd7kfuuh@work> <20200611103709.GB19132@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200611103709.GB19132@quack2.suse.cz> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Thu, Jun 11, 2020 at 12:37:09PM +0200, Jan Kara wrote: > 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. Ok, thanks. I'll prepare a patch. Regards, -Lukas > > Honza > -- > Jan Kara > SUSE Labs, CR >