From patchwork Mon Jan 11 13:02:29 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kroah-Hartman X-Patchwork-Id: 361070 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=-18.8 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, 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 22F39C433E6 for ; Mon, 11 Jan 2021 13:19:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E63EF20771 for ; Mon, 11 Jan 2021 13:19:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733276AbhAKNT3 (ORCPT ); Mon, 11 Jan 2021 08:19:29 -0500 Received: from mail.kernel.org ([198.145.29.99]:37792 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729395AbhAKNT1 (ORCPT ); Mon, 11 Jan 2021 08:19:27 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 1696622AAF; Mon, 11 Jan 2021 13:18:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1610371126; bh=DlM0LE88xUdrg3zLOdefm+q9D1U5twwMobrnzwH5+ws=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=qEI3a1eaXvAr9R9KLMp/jqdinBkB24XJp2lcUK+3RRbDMFfX7lmOL0Iw9kmtF3CrW CX2I8mRm8mf9K0Zj3jf+UCz45c8PLWSBv6Yl5/xsslVmnk8wYGAjxFFgrig/rZYyTW PFDLeZTsKz00XNFNGK6B+Mmc8C+6/KzFGxygbu9I= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Filipe Manana , Qu Wenruo , David Sterba Subject: [PATCH 5.10 125/145] btrfs: qgroup: dont try to wait flushing if were already holding a transaction Date: Mon, 11 Jan 2021 14:02:29 +0100 Message-Id: <20210111130054.514634756@linuxfoundation.org> X-Mailer: git-send-email 2.30.0 In-Reply-To: <20210111130048.499958175@linuxfoundation.org> References: <20210111130048.499958175@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Qu Wenruo commit ae5e070eaca9dbebde3459dd8f4c2756f8c097d0 upstream. There is a chance of racing for qgroup flushing which may lead to deadlock: Thread A | Thread B (not holding trans handle) | (holding a trans handle) --------------------------------+-------------------------------- __btrfs_qgroup_reserve_meta() | __btrfs_qgroup_reserve_meta() |- try_flush_qgroup() | |- try_flush_qgroup() |- QGROUP_FLUSHING bit set | | | | |- test_and_set_bit() | | |- wait_event() |- btrfs_join_transaction() | |- btrfs_commit_transaction()| !!! DEAD LOCK !!! Since thread A wants to commit transaction, but thread B is holding a transaction handle, blocking the commit. At the same time, thread B is waiting for thread A to finish its commit. This is just a hot fix, and would lead to more EDQUOT when we're near the qgroup limit. The proper fix would be to make all metadata/data reservations happen without holding a transaction handle. CC: stable@vger.kernel.org # 5.9+ Reviewed-by: Filipe Manana Signed-off-by: Qu Wenruo Signed-off-by: David Sterba Signed-off-by: Greg Kroah-Hartman --- fs/btrfs/qgroup.c | 30 ++++++++++++++++++++---------- 1 file changed, 20 insertions(+), 10 deletions(-) --- a/fs/btrfs/qgroup.c +++ b/fs/btrfs/qgroup.c @@ -3565,16 +3565,6 @@ static int try_flush_qgroup(struct btrfs bool can_commit = true; /* - * We don't want to run flush again and again, so if there is a running - * one, we won't try to start a new flush, but exit directly. - */ - if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) { - wait_event(root->qgroup_flush_wait, - !test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)); - return 0; - } - - /* * If current process holds a transaction, we shouldn't flush, as we * assume all space reservation happens before a transaction handle is * held. @@ -3588,6 +3578,26 @@ static int try_flush_qgroup(struct btrfs current->journal_info != BTRFS_SEND_TRANS_STUB) can_commit = false; + /* + * We don't want to run flush again and again, so if there is a running + * one, we won't try to start a new flush, but exit directly. + */ + if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) { + /* + * We are already holding a transaction, thus we can block other + * threads from flushing. So exit right now. This increases + * the chance of EDQUOT for heavy load and near limit cases. + * But we can argue that if we're already near limit, EDQUOT is + * unavoidable anyway. + */ + if (!can_commit) + return 0; + + wait_event(root->qgroup_flush_wait, + !test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)); + return 0; + } + ret = btrfs_start_delalloc_snapshot(root); if (ret < 0) goto out;