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=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 924F0C433EF for ; Mon, 6 Sep 2021 12:27:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72F9D61039 for ; Mon, 6 Sep 2021 12:27:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242258AbhIFM26 (ORCPT ); Mon, 6 Sep 2021 08:28:58 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:56720 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242247AbhIFM25 (ORCPT ); Mon, 6 Sep 2021 08:28:57 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id E5CB41FEEE; Mon, 6 Sep 2021 12:27:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1630931270; h=from:from:reply-to:reply-to: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=q5Gjv/o2r31Pg5vgSEoAXdlHZThs4FawYEtj69ES0+s=; b=Wjec5BuFH/snRU+lj0EWXJqw1F4PdiMVA37DDVriVRPiWzNFBLTMKM+So6qgjp8cBOY/qw mLKxW4bglIuDqnshO4dHOIsK58LZE/JEXPlTsU3dQChorU5/ge+QzD9/+bwdt4nGYB2374 Ttk8f5ucNkvnpjn9swZWaWNBoOv0zCM= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1630931270; h=from:from:reply-to:reply-to: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=q5Gjv/o2r31Pg5vgSEoAXdlHZThs4FawYEtj69ES0+s=; b=y8Fm6tjaBOyin7BFqKGEf25V2DvFHbrdu1O/xm6qRqlDTwZCeG5RsNRnYtAbEYpos5US/D R0+7g9L9iR25S/Cw== Received: from ds.suse.cz (ds.suse.cz [10.100.12.205]) by relay2.suse.de (Postfix) with ESMTP id DB451A3B88; Mon, 6 Sep 2021 12:27:50 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 83674DA781; Mon, 6 Sep 2021 14:27:47 +0200 (CEST) Date: Mon, 6 Sep 2021 14:27:47 +0200 From: David Sterba To: Baptiste Lepers Cc: "Paul E . McKenney" , Chris Mason , Josef Bacik , David Sterba , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans Message-ID: <20210906122747.GG3379@suse.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Baptiste Lepers , "Paul E . McKenney" , Chris Mason , Josef Bacik , David Sterba , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210906012559.8605-1-baptiste.lepers@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210906012559.8605-1-baptiste.lepers@gmail.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Mon, Sep 06, 2021 at 11:25:59AM +1000, Baptiste Lepers wrote: > Per comment, record_root_in_trans orders the writes of the root->state > and root->last_trans: > set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state); > smp_wmb(); > root->last_trans = trans->transid; > > But the barrier that enforces the order on the read side is misplaced: > smp_rmb(); <-- misplaced > if (root->last_trans == trans->transid && > <-- missing barrier here --> > !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state)) > > This patches fixes the ordering and wraps the racy accesses with > READ_ONCE and WRITE_ONCE calls to avoid load/store tearing. > > Fixes: 7585717f304f5 ("Btrfs: fix relocation races") > Signed-off-by: Baptiste Lepers > --- > fs/btrfs/transaction.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c > index 14b9fdc8aaa9..a609222e6704 100644 > --- a/fs/btrfs/transaction.c > +++ b/fs/btrfs/transaction.c > @@ -437,7 +437,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans, > (unsigned long)root->root_key.objectid, > BTRFS_ROOT_TRANS_TAG); > spin_unlock(&fs_info->fs_roots_radix_lock); > - root->last_trans = trans->transid; > + WRITE_ONCE(root->last_trans, trans->transid); > > /* this is pretty tricky. We don't want to > * take the relocation lock in btrfs_record_root_in_trans > @@ -489,7 +489,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans, > struct btrfs_root *root) > { > struct btrfs_fs_info *fs_info = root->fs_info; > - int ret; > + int ret, last_trans; > > if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state)) > return 0; > @@ -498,8 +498,9 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans, > * see record_root_in_trans for comments about IN_TRANS_SETUP usage > * and barriers > */ > + last_trans = READ_ONCE(root->last_trans); > smp_rmb(); > - if (root->last_trans == trans->transid && > + if (last_trans == trans->transid && > !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state)) Aren't the smp_rmb barriers supposed to be used before the condition?