From patchwork Thu Sep 10 10:30:41 2015 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Filipe Manana X-Patchwork-Id: 7153311 Return-Path: X-Original-To: patchwork-linux-btrfs@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 395369F314 for ; Thu, 10 Sep 2015 10:31:00 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 548812066B for ; Thu, 10 Sep 2015 10:30:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 59CD920656 for ; Thu, 10 Sep 2015 10:30:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753550AbbIJKav (ORCPT ); Thu, 10 Sep 2015 06:30:51 -0400 Received: from mail.kernel.org ([198.145.29.136]:36761 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752558AbbIJKat (ORCPT ); Thu, 10 Sep 2015 06:30:49 -0400 Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 405C02066B; Thu, 10 Sep 2015 10:30:48 +0000 (UTC) Received: from debian3.lan (bl8-199-62.dsl.telepac.pt [85.241.199.62]) (using TLSv1.2 with cipher AES128-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id CE9D420665; Thu, 10 Sep 2015 10:30:46 +0000 (UTC) From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Cc: Filipe Manana Subject: [PATCH] Btrfs: remove unnecessary locking of cleaner_mutex to avoid deadlock Date: Thu, 10 Sep 2015 11:30:41 +0100 Message-Id: <1441881041-10992-1-git-send-email-fdmanana@kernel.org> X-Mailer: git-send-email 2.1.3 X-Spam-Status: No, score=-6.9 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_HI, T_RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Filipe Manana After commmit e44163e17796 ("btrfs: explictly delete unused block groups in close_ctree and ro-remount"), added in the 4.3 merge window, we have calls to btrfs_delete_unused_bgs() while holding the cleaner_mutex. This can cause a deadlock with a concurrent block group relocation (when a filesystem balance or shrink operation is in progress for example) because btrfs_delete_unused_bgs() locks delete_unused_bgs_mutex and the relocation path locks first delete_unused_bgs_mutex and then it locks cleaner_mutex, resulting in a classic ABBA deadlock: CPU 0 CPU 1 lock fs_info->cleaner_mutex __btrfs_balance() || btrfs_shrink_device() lock fs_info->delete_unused_bgs_mutex btrfs_relocate_chunk() btrfs_relocate_block_group() lock fs_info->cleaner_mutex btrfs_delete_unused_bgs() lock fs_info->delete_unused_bgs_mutex Fix this by not taking the cleaner_mutex before calling btrfs_delete_unused_bgs() because it's no longer needed after commit 67c5e7d464bc ("Btrfs: fix race between balance and unused block group deletion"). The mutex fs_info->delete_unused_bgs_mutex, the spinlock fs_info->unused_bgs_lock and a block group's spinlock are enough to get correct serialization between tasks running relocation and unused block group deletion (as well as between multiple tasks concurrently calling btrfs_delete_unused_bgs()). This issue was discussed (in the mailing list) during the review of the patch titled "btrfs: explictly delete unused block groups in close_ctree and ro-remount" and it was agreed that acquiring the cleaner mutex had to be dropped after the patch titled "Btrfs: fix race between balance and unused block group deletion" got merged (both patches were submitted at about the same time, but one landed in kernel 4.2 and the other in the 4.3 merge window). Signed-off-by: Filipe Manana --- fs/btrfs/disk-io.c | 2 -- fs/btrfs/super.c | 2 -- 2 files changed, 4 deletions(-) diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 0b658d0..aa59871 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -3762,9 +3762,7 @@ void close_ctree(struct btrfs_root *root) * block groups queued for removal, the deletion will be * skipped when we quit the cleaner thread. */ - mutex_lock(&root->fs_info->cleaner_mutex); btrfs_delete_unused_bgs(root->fs_info); - mutex_unlock(&root->fs_info->cleaner_mutex); ret = btrfs_commit_super(root); if (ret) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index c389c13..5a186d7 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -1658,9 +1658,7 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data) * groups on disk until we're mounted read-write again * unless we clean them up here. */ - mutex_lock(&root->fs_info->cleaner_mutex); btrfs_delete_unused_bgs(fs_info); - mutex_unlock(&root->fs_info->cleaner_mutex); btrfs_dev_replace_suspend_for_unmount(fs_info); btrfs_scrub_cancel(fs_info);