From patchwork Tue Sep 24 09:49:54 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Filipe Manana X-Patchwork-Id: 11158361 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id A9DE4912 for ; Tue, 24 Sep 2019 09:50:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 756B220872 for ; Tue, 24 Sep 2019 09:50:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1569318600; bh=Zg1jxdfhYnSHtmNw7o42pr55qxTRACpuA8Z8Jb+JtD0=; h=From:To:Subject:Date:List-ID:From; b=K6i8jeK6Ybwzu5MAw+yiro6rqAHOFMcjORX/sAdKQq26M/HqL9Uq8iuLRCZuQmnNU o7LGrOczyx/1XCBfGOfSZImcehcaT58Qvn1N4Hs9ASPV7zKVTpd/I5bUq9g6xefm/Z AeXbnheqQxAJrUPkxmTuC/X8ZnlXkv2R9TS44P9g= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2395400AbfIXJt7 (ORCPT ); Tue, 24 Sep 2019 05:49:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:39076 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2504383AbfIXJt6 (ORCPT ); Tue, 24 Sep 2019 05:49:58 -0400 Received: from localhost.localdomain (bl8-197-74.dsl.telepac.pt [85.241.197.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B29F920872 for ; Tue, 24 Sep 2019 09:49:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1569318597; bh=Zg1jxdfhYnSHtmNw7o42pr55qxTRACpuA8Z8Jb+JtD0=; h=From:To:Subject:Date:From; b=uewu4CN1lcSzIc+T5BVTVb14ciLWzttWsi44nVIMCykgAC0nNlt4WZvLy6C38Gf8a VXePxBVabKUPxM4qFZtSehPeCDRbwRot1PPG5sozcZdkFar2ayCWyKuIgmwB1VUR20 Az94Myjo5tDnAlGwPSBOfApF/0pydGouB5jsMCw4= From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Subject: [PATCH] Btrfs: fix race setting up and completing qgroup rescan workers Date: Tue, 24 Sep 2019 10:49:54 +0100 Message-Id: <20190924094954.1304-1-fdmanana@kernel.org> X-Mailer: git-send-email 2.11.0 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org From: Filipe Manana There is a race between setting up a qgroup rescan worker and completing a qgroup rescan worker that can lead to callers of the qgroup rescan wait ioctl to either not wait for the rescan worker to complete or to hang forever due to missing wake ups. The following diagram shows a sequence of steps that illustrates the race. CPU 1 CPU 2 CPU 3 btrfs_ioctl_quota_rescan() btrfs_qgroup_rescan() qgroup_rescan_init() mutex_lock(&fs_info->qgroup_rescan_lock) spin_lock(&fs_info->qgroup_lock) fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN init_completion( &fs_info->qgroup_rescan_completion) fs_info->qgroup_rescan_running = true mutex_unlock(&fs_info->qgroup_rescan_lock) spin_unlock(&fs_info->qgroup_lock) btrfs_init_work() --> starts the worker btrfs_qgroup_rescan_worker() mutex_lock(&fs_info->qgroup_rescan_lock) fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN mutex_unlock(&fs_info->qgroup_rescan_lock) starts transaction, updates qgroup status item, etc btrfs_ioctl_quota_rescan() btrfs_qgroup_rescan() qgroup_rescan_init() mutex_lock(&fs_info->qgroup_rescan_lock) spin_lock(&fs_info->qgroup_lock) fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN init_completion( &fs_info->qgroup_rescan_completion) fs_info->qgroup_rescan_running = true mutex_unlock(&fs_info->qgroup_rescan_lock) spin_unlock(&fs_info->qgroup_lock) btrfs_init_work() --> starts another worker mutex_lock(&fs_info->qgroup_rescan_lock) fs_info->qgroup_rescan_running = false mutex_unlock(&fs_info->qgroup_rescan_lock) complete_all(&fs_info->qgroup_rescan_completion) Before the rescan worker started by the task at CPU 3 completes, if another task calls btrfs_ioctl_quota_rescan(), it will get -EINPROGRESS because the flag BTRFS_QGROUP_STATUS_FLAG_RESCAN is set at fs_info->qgroup_flags, which is expected and correct behaviour. However if other task calls btrfs_ioctl_quota_rescan_wait() before the rescan worker started by the task at CPU 3 completes, it will return immediately without waiting for the new rescan worker to complete, because fs_info->qgroup_rescan_running is set to false by CPU 2. This race is making test case btrfs/171 (from fstests) to fail often: btrfs/171 9s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad) --- tests/btrfs/171.out 2018-09-16 21:30:48.505104287 +0100 +++ /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad 2019-09-19 02:01:36.938486039 +0100 @@ -1,2 +1,3 @@ QA output created by 171 +ERROR: quota rescan failed: Operation now in progress Silence is golden ... (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/171.out /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad' to see the entire diff) That is because the test calls the btrfs-progs commands "qgroup quota rescan -w", "qgroup assign" and "qgroup remove" in a sequence that makes calls to the rescan start ioctl fail with -EINPROGRESS (note the "btrfs" commands 'qgroup assign' and 'qgroup remove' often call the rescan start ioctl after calling the qgroup assign ioctl, btrfs_ioctl_qgroup_assign()), since previous waits didn't actually wait for a rescan worker to complete. Another problem the race can cause is missing wake ups for waiters, since the call to complete_all() happens outside a critical section and after clearing the flag BTRFS_QGROUP_STATUS_FLAG_RESCAN. In the sequence diagram above, if we have a waiter for the first rescan task (executed by CPU 2), then fs_info->qgroup_rescan_completion.wait is not empty, and if after the rescan worker clears BTRFS_QGROUP_STATUS_FLAG_RESCAN and before it calls complete_all() against fs_info->qgroup_rescan_completion, the task at CPU 3 calls init_completion() against fs_info->qgroup_rescan_completion which re-initilizes its wait queue to an empty queue, therefore causing the rescan worker at CPU 2 to call complete_all() against an empty queue, never waking up the task waiting for that rescan worker. Fix this by clearing BTRFS_QGROUP_STATUS_FLAG_RESCAN and setting fs_info->qgroup_rescan_running to false in the same critical section, delimited by the mutex fs_info->qgroup_rescan_lock, as well as doing the call to complete_all() in that same critical section. This gives the protection needed to avoid rescan wait ioctl callers not waiting for a running rescan worker and the lost wake ups problem, since setting that rescan flag and boolean as well as initializing the wait queue is done already in a critical section delimited by that mutex (at qgroup_rescan_init()). Fixes: 57254b6ebce4ce ("Btrfs: add ioctl to wait for qgroup rescan completion") Fixes: d2c609b834d62f ("btrfs: properly track when rescan worker is running") Signed-off-by: Filipe Manana Reviewed-by: Josef Bacik --- fs/btrfs/qgroup.c | 33 +++++++++++++++++++-------------- 1 file changed, 19 insertions(+), 14 deletions(-) diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c index 8d3bd799ac7d..52701c1be109 100644 --- a/fs/btrfs/qgroup.c +++ b/fs/btrfs/qgroup.c @@ -3166,9 +3166,6 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work) btrfs_free_path(path); mutex_lock(&fs_info->qgroup_rescan_lock); - if (!btrfs_fs_closing(fs_info)) - fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN; - if (err > 0 && fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT) { fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT; @@ -3184,16 +3181,30 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work) trans = btrfs_start_transaction(fs_info->quota_root, 1); if (IS_ERR(trans)) { err = PTR_ERR(trans); + trans = NULL; btrfs_err(fs_info, "fail to start transaction for status update: %d", err); - goto done; } - ret = update_qgroup_status_item(trans); - if (ret < 0) { - err = ret; - btrfs_err(fs_info, "fail to update qgroup status: %d", err); + + mutex_lock(&fs_info->qgroup_rescan_lock); + if (!btrfs_fs_closing(fs_info)) + fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN; + if (trans) { + ret = update_qgroup_status_item(trans); + if (ret < 0) { + err = ret; + btrfs_err(fs_info, "fail to update qgroup status: %d", + err); + } } + fs_info->qgroup_rescan_running = false; + complete_all(&fs_info->qgroup_rescan_completion); + mutex_unlock(&fs_info->qgroup_rescan_lock); + + if (!trans) + return; + btrfs_end_transaction(trans); if (btrfs_fs_closing(fs_info)) { @@ -3204,12 +3215,6 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work) } else { btrfs_err(fs_info, "qgroup scan failed with %d", err); } - -done: - mutex_lock(&fs_info->qgroup_rescan_lock); - fs_info->qgroup_rescan_running = false; - mutex_unlock(&fs_info->qgroup_rescan_lock); - complete_all(&fs_info->qgroup_rescan_completion); } /*