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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY, 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 78A3CC433E0 for ; Thu, 18 Jun 2020 15:23:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 49AE62075E for ; Thu, 18 Jun 2020 15:23:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="wIXZR245" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729900AbgFRPXF (ORCPT ); Thu, 18 Jun 2020 11:23:05 -0400 Received: from userp2130.oracle.com ([156.151.31.86]:53102 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728269AbgFRPXE (ORCPT ); Thu, 18 Jun 2020 11:23:04 -0400 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 05IFLs3J065365; Thu, 18 Jun 2020 15:22:59 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to; s=corp-2020-01-29; bh=eNWfmZdmYXiX9iEWGc/gueYpgw2Nhm+0CuXBMAUoTzQ=; b=wIXZR245s7LeBjWPKEu7t7v1oJWf6q+0hI/AIZddWx2b4HDKDTKqsD1LG+xh6mQdMVuf 52CyI5MK6bIi7tW4TF37c50hAfiVp/jpURRlc+hOYs3JH5FxDCFkRzISnP2U/ncbDIAt 6riGGd4hMpcqr6VgCV3WjXF76pwSyM8959gM2d065pEPuvbjHMO7VLy9hX6SZro4JkR2 U8JS28PQUNVnorHGVmaeSF5hyeDAEVUzMaPeZI8j0phWOyFqc/ENIxIcwSpr7c790FLd uQQvsiq6WvWpvxrOgf1fBNKKP6ceOeISihmN0VQ6/JAzAPlct+479JixCFuB7B+FKNkV fw== Received: from aserp3020.oracle.com (aserp3020.oracle.com [141.146.126.70]) by userp2130.oracle.com with ESMTP id 31q6601uqu-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 18 Jun 2020 15:22:59 +0000 Received: from pps.filterd (aserp3020.oracle.com [127.0.0.1]) by aserp3020.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 05IFJFms136555; Thu, 18 Jun 2020 15:20:58 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3020.oracle.com with ESMTP id 31q66sgkt6-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 18 Jun 2020 15:20:58 +0000 Received: from abhmp0020.oracle.com (abhmp0020.oracle.com [141.146.116.26]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 05IFKqLe017468; Thu, 18 Jun 2020 15:20:52 GMT Received: from localhost (/67.169.218.210) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 18 Jun 2020 08:20:52 -0700 Date: Thu, 18 Jun 2020 08:20:51 -0700 From: "Darrick J. Wong" To: Waiman Long Cc: linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, Dave Chinner , Qian Cai , Eric Sandeen Subject: Re: [PATCH v3] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim Message-ID: <20200618152051.GU11245@magnolia> References: <20200618150557.23741-1-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200618150557.23741-1-longman@redhat.com> X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9655 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 mlxlogscore=999 bulkscore=0 adultscore=0 phishscore=0 suspectscore=1 mlxscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006180116 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9655 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 lowpriorityscore=0 malwarescore=0 bulkscore=0 phishscore=0 adultscore=0 priorityscore=1501 mlxscore=0 spamscore=0 clxscore=1015 mlxlogscore=999 suspectscore=1 impostorscore=0 cotscore=-2147483648 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006180117 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Thu, Jun 18, 2020 at 11:05:57AM -0400, Waiman Long wrote: > Depending on the workloads, the following circular locking dependency > warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo > lock) may show up: > > ====================================================== > WARNING: possible circular locking dependency detected > 5.0.0-rc1+ #60 Tainted: G W > ------------------------------------------------------ > fsfreeze/4346 is trying to acquire lock: > 0000000026f1d784 (fs_reclaim){+.+.}, at: > fs_reclaim_acquire.part.19+0x5/0x30 > > but task is already holding lock: > 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650 > > which lock already depends on the new lock. > : > Possible unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(sb_internal); > lock(fs_reclaim); > lock(sb_internal); > lock(fs_reclaim); > > *** DEADLOCK *** > > 4 locks held by fsfreeze/4346: > #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650 > #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at: freeze_super+0xda/0x290 > #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650 > #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650 > > stack backtrace: > Call Trace: > dump_stack+0xe0/0x19a > print_circular_bug.isra.10.cold.34+0x2f4/0x435 > check_prev_add.constprop.19+0xca1/0x15f0 > validate_chain.isra.14+0x11af/0x3b50 > __lock_acquire+0x728/0x1200 > lock_acquire+0x269/0x5a0 > fs_reclaim_acquire.part.19+0x29/0x30 > fs_reclaim_acquire+0x19/0x20 > kmem_cache_alloc+0x3e/0x3f0 > kmem_zone_alloc+0x79/0x150 > xfs_trans_alloc+0xfa/0x9d0 > xfs_sync_sb+0x86/0x170 > xfs_log_sbcount+0x10f/0x140 > xfs_quiesce_attr+0x134/0x270 > xfs_fs_freeze+0x4a/0x70 > freeze_super+0x1af/0x290 > do_vfs_ioctl+0xedc/0x16c0 > ksys_ioctl+0x41/0x80 > __x64_sys_ioctl+0x73/0xa9 > do_syscall_64+0x18f/0xd23 > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > This is a false positive as all the dirty pages are flushed out before > the filesystem can be frozen. > > One way to avoid this splat is to add GFP_NOFS to the affected allocation > calls. This is what PF_MEMALLOC_NOFS per-process flag is for. This does > reduce the potential source of memory where reclaim can be done. This > shouldn't really matter unless the system is really running out of > memory. In that particular case, the filesystem freeze operation may > fail while it was succeeding previously. > > Without this patch, the command sequence below will show that the lock > dependency chain sb_internal -> fs_reclaim exists. > > # fsfreeze -f /home > # fsfreeze --unfreeze /home > # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal > > After applying the patch, such sb_internal -> fs_reclaim lock dependency > chain can no longer be found. Because of that, the locking dependency > warning will not be shown. > > Suggested-by: Dave Chinner > Signed-off-by: Waiman Long > --- > fs/xfs/xfs_super.c | 24 +++++++++++++++++++++++- > 1 file changed, 23 insertions(+), 1 deletion(-) > > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c > index 379cbff438bc..6a95c82f2f1b 100644 > --- a/fs/xfs/xfs_super.c > +++ b/fs/xfs/xfs_super.c > @@ -913,11 +913,33 @@ xfs_fs_freeze( > struct super_block *sb) > { > struct xfs_mount *mp = XFS_M(sb); > + unsigned long pflags; > + int ret; Minor nit: please indent the variable names to line up with *sb/*mp. Otherwise this seems reasoanble. --D > > + /* > + * A fs_reclaim pseudo lock is added to check for potential deadlock > + * condition with fs reclaim. The following lockdep splat was hit > + * occasionally. This is actually a false positive as the allocation > + * is being done only after the frozen filesystem is no longer dirty. > + * One way to avoid this splat is to add GFP_NOFS to the affected > + * allocation calls. This is what PF_MEMALLOC_NOFS is for. > + * > + * CPU0 CPU1 > + * ---- ---- > + * lock(sb_internal); > + * lock(fs_reclaim); > + * lock(sb_internal); > + * lock(fs_reclaim); > + * > + * *** DEADLOCK *** > + */ > + current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS); > xfs_stop_block_reaping(mp); > xfs_save_resvblks(mp); > xfs_quiesce_attr(mp); > - return xfs_sync_sb(mp, true); > + ret = xfs_sync_sb(mp, true); > + current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS); > + return ret; > } > > STATIC int > -- > 2.18.1 >