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=-20.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 2015EC64E7B for ; Mon, 30 Nov 2020 01:20:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE7FB2076A for ; Mon, 30 Nov 2020 01:20:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727743AbgK3BUP (ORCPT ); Sun, 29 Nov 2020 20:20:15 -0500 Received: from szxga05-in.huawei.com ([45.249.212.191]:8529 "EHLO szxga05-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726510AbgK3BUO (ORCPT ); Sun, 29 Nov 2020 20:20:14 -0500 Received: from DGGEMS403-HUB.china.huawei.com (unknown [172.30.72.60]) by szxga05-in.huawei.com (SkyGuard) with ESMTP id 4CknSW5LHTzhk9G; Mon, 30 Nov 2020 09:19:07 +0800 (CST) Received: from [10.136.114.67] (10.136.114.67) by smtp.huawei.com (10.3.19.203) with Microsoft SMTP Server (TLS) id 14.3.487.0; Mon, 30 Nov 2020 09:19:26 +0800 Subject: Re: [f2fs-dev] [PATCH] f2fs: Fix deadlock between f2fs_quota_sync and block_operation To: Shachar Raindel , , , , , CC: , References: <20201128174124.22397-1-shacharr@gmail.com> From: Chao Yu Message-ID: Date: Mon, 30 Nov 2020 09:19:26 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20201128174124.22397-1-shacharr@gmail.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.136.114.67] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/11/29 1:41, Shachar Raindel wrote: > This deadlock is hitting Android users (Pixel 3/3a/4) with Magisk, due > to frequent umount/mount operations that trigger quota_sync, hitting > the race. See https://github.com/topjohnwu/Magisk/issues/3171 for > additional impact discussion. > > In commit db6ec53b7e03, we added a semaphore to protect quota flags. > As part of this commit, we changed f2fs_quota_sync to call > f2fs_lock_op, in an attempt to prevent an AB/BA type deadlock with > quota_sem locking in block_operation. However, rwsem in Linux is not > recursive. Therefore, the following deadlock can occur: > > f2fs_quota_sync > down_read(cp_rwsem) // f2fs_lock_op > filemap_fdatawrite > f2fs_write_data_pages > ... > block_opertaion > down_write(cp_rwsem) - marks rwsem as > "writer pending" > down_read_trylock(cp_rwsem) - fails as there is > a writer pending. > Code keeps on trying, > live-locking the filesystem. f2fs_write_single_data_page() will not grab read lock of cp_rwsem now, could you please check f2fs code in mainline? /* Dentry/quota blocks are controlled by checkpoint */ if (S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) { /* * We need to wait for node_write to avoid block allocation during * checkpoint. This can only happen to quota writes which can cause * the below discard race condition. */ if (IS_NOQUOTA(inode)) down_read(&sbi->node_write); fio.need_lock = LOCK_DONE; err = f2fs_do_write_data_page(&fio); if (IS_NOQUOTA(inode)) up_read(&sbi->node_write); goto done; } Thanks, > > We solve this by creating a new rwsem, used specifically to > synchronize this case, instead of attempting to reuse an existing > lock. > > Signed-off-by: Shachar Raindel > > Fixes: db6ec53b7e03 f2fs: add a rw_sem to cover quota flag changes > --- > fs/f2fs/f2fs.h | 3 +++ > fs/f2fs/super.c | 13 +++++++++++-- > 2 files changed, 14 insertions(+), 2 deletions(-) > > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h > index cb700d797296..b3e55137be7f 100644 > --- a/fs/f2fs/f2fs.h > +++ b/fs/f2fs/f2fs.h > @@ -1448,6 +1448,7 @@ struct f2fs_sb_info { > struct inode *meta_inode; /* cache meta blocks */ > struct mutex cp_mutex; /* checkpoint procedure lock */ > struct rw_semaphore cp_rwsem; /* blocking FS operations */ > + struct rw_semaphore cp_quota_rwsem; /* blocking quota sync operations */ > struct rw_semaphore node_write; /* locking node writes */ > struct rw_semaphore node_change; /* locking node change */ > wait_queue_head_t cp_wait; > @@ -1961,12 +1962,14 @@ static inline void f2fs_unlock_op(struct f2fs_sb_info *sbi) > > static inline void f2fs_lock_all(struct f2fs_sb_info *sbi) > { > + down_write(&sbi->cp_quota_rwsem); > down_write(&sbi->cp_rwsem); > } > > static inline void f2fs_unlock_all(struct f2fs_sb_info *sbi) > { > up_write(&sbi->cp_rwsem); > + up_write(&sbi->cp_quota_rwsem); > } > > static inline int __get_cp_reason(struct f2fs_sb_info *sbi) > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c > index 00eff2f51807..5ce61147d7e5 100644 > --- a/fs/f2fs/super.c > +++ b/fs/f2fs/super.c > @@ -2209,8 +2209,16 @@ int f2fs_quota_sync(struct super_block *sb, int type) > * f2fs_dquot_commit > * block_operation > * down_read(quota_sem) > + * > + * However, we cannot use the cp_rwsem to prevent this > + * deadlock, as the cp_rwsem is taken for read inside the > + * f2fs_dquot_commit code, and rwsem is not recursive. > + * > + * We therefore use a special lock to synchronize > + * f2fs_quota_sync with block_operations, as this is the only > + * place where such recursion occurs. > */ > - f2fs_lock_op(sbi); > + down_read(&sbi->cp_quota_rwsem); > > down_read(&sbi->quota_sem); > ret = dquot_writeback_dquots(sb, type); > @@ -2251,7 +2259,7 @@ int f2fs_quota_sync(struct super_block *sb, int type) > if (ret) > set_sbi_flag(F2FS_SB(sb), SBI_QUOTA_NEED_REPAIR); > up_read(&sbi->quota_sem); > - f2fs_unlock_op(sbi); > + up_read(&sbi->cp_quota_rwsem); > return ret; > } > > @@ -3599,6 +3607,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent) > > init_rwsem(&sbi->cp_rwsem); > init_rwsem(&sbi->quota_sem); > + init_rwsem(&sbi->cp_quota_rwsem); > init_waitqueue_head(&sbi->cp_wait); > init_sb_info(sbi); > >