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=-18.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 56C2FC47082 for ; Tue, 8 Jun 2021 23:04:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3898C60FEE for ; Tue, 8 Jun 2021 23:04:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235146AbhFHXGr (ORCPT ); Tue, 8 Jun 2021 19:06:47 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:23112 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235062AbhFHXGq (ORCPT ); Tue, 8 Jun 2021 19:06:46 -0400 Received: from pps.filterd (m0109331.ppops.net [127.0.0.1]) by mx0a-00082601.pphosted.com (8.16.0.43/8.16.0.43) with SMTP id 158Mvmsx015244 for ; Tue, 8 Jun 2021 16:04:52 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : in-reply-to : references : mime-version : content-transfer-encoding : content-type; s=facebook; bh=mfav6jzNKGWoxXgqLsimDL5+HxKJWmRmET2uHflSm9w=; b=OjqfaN2Os6DxSL7HmOn4jDtij1QwyZYPxFDhWgbljrVVB85ZPP7F2i1BRuaLZ7Rrp80u F9+mfkNCMARlB+MSC3rh4vz7UjDseYvOEtUZKn8LwPjakhCg/wlUY8oB9jdWmd3tnEb2 i0qc72fz4JnRtBJRCP8a4Pp2Nog77WtweDM= Received: from maileast.thefacebook.com ([163.114.130.16]) by mx0a-00082601.pphosted.com with ESMTP id 391rhyrkvd-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128 verify=NOT) for ; Tue, 08 Jun 2021 16:04:52 -0700 Received: from intmgw003.48.prn1.facebook.com (2620:10d:c0a8:1b::d) by mail.thefacebook.com (2620:10d:c0a8:82::f) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Tue, 8 Jun 2021 16:04:51 -0700 Received: by devvm3388.prn0.facebook.com (Postfix, from userid 111017) id 3519582542C9; Tue, 8 Jun 2021 16:02:28 -0700 (PDT) From: Roman Gushchin To: Andrew Morton , Tejun Heo CC: , , , Alexander Viro , Jan Kara , Dennis Zhou , Dave Chinner , , Roman Gushchin Subject: [PATCH v9 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount() Date: Tue, 8 Jun 2021 16:02:19 -0700 Message-ID: <20210608230225.2078447-3-guro@fb.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210608230225.2078447-1-guro@fb.com> References: <20210608230225.2078447-1-guro@fb.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-FB-Internal: Safe Content-Type: text/plain X-Proofpoint-GUID: AbjGz8O_o22x7eYzbkV47eALL5zk9vun X-Proofpoint-ORIG-GUID: AbjGz8O_o22x7eYzbkV47eALL5zk9vun X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-06-08_17:2021-06-04,2021-06-08 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxlogscore=726 priorityscore=1501 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 adultscore=0 phishscore=0 mlxscore=0 malwarescore=0 impostorscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2106080145 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org A full memory barrier is required between clearing SB_ACTIVE flag in generic_shutdown_super() and checking isw_nr_in_flight in cgroup_writeback_umount(), otherwise a new switch operation might be scheduled after atomic_read(&isw_nr_in_flight) returned 0. This would result in a non-flushed isw_wq, and a potential crash. The problem hasn't yet been seen in the real life and was discovered by Jan Kara by looking into the code. Suggested-by: Jan Kara Signed-off-by: Roman Gushchin Reviewed-by: Jan Kara --- fs/fs-writeback.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 7d2891d7ac12..b6fc13a4962d 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1000,6 +1000,12 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_i= d, unsigned long nr, */ void cgroup_writeback_umount(void) { + /* + * SB_ACTIVE should be reliably cleared before checking + * isw_nr_in_flight, see generic_shutdown_super(). + */ + smp_mb(); + if (atomic_read(&isw_nr_in_flight)) { /* * Use rcu_barrier() to wait for all pending callbacks to --=20 2.31.1