From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-btrfs-owner@kernel.org>
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=-12.2 required=3.0 tests=BAYES_00,DKIM_SIGNED,
	DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,
	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 88A97C432BE
	for <linux-btrfs@archiver.kernel.org>; Thu,  2 Sep 2021 15:28:23 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [23.128.96.18])
	by mail.kernel.org (Postfix) with ESMTP id 6E8C561074
	for <linux-btrfs@archiver.kernel.org>; Thu,  2 Sep 2021 15:28:23 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1345781AbhIBP3U (ORCPT <rfc822;linux-btrfs@archiver.kernel.org>);
        Thu, 2 Sep 2021 11:29:20 -0400
Received: from smtp-out1.suse.de ([195.135.220.28]:37088 "EHLO
        smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1345689AbhIBP3Q (ORCPT
        <rfc822;linux-btrfs@vger.kernel.org>); Thu, 2 Sep 2021 11:29:16 -0400
Received: from relay2.suse.de (relay2.suse.de [149.44.160.134])
        by smtp-out1.suse.de (Postfix) with ESMTP id 4BB9B225BF;
        Thu,  2 Sep 2021 15:28:17 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa;
        t=1630596497;
        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=uOiLVx6DDpiOVqwzmgO67c4gRx1DIEl6pnHLlVAZWA0=;
        b=ftKBrsIgQG7rq1l0R0WnAtdo5aycHTEt8b6Gyp39lhJWtKNCLCODPgoFEmo/Rkmr8q3SNt
        qKn+MfIfl2LHKzVorRxkv5dps153/oSeICB3rlOtmZjgeFZ4Gij6UwBkMXAEgIE0Ar29yQ
        GPtgu2B1p/fcA3WeHRvUicvTkld0E28=
DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz;
        s=susede2_ed25519; t=1630596497;
        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=uOiLVx6DDpiOVqwzmgO67c4gRx1DIEl6pnHLlVAZWA0=;
        b=wtJNX6mfDUpfaorDo1rvRiQcso6lrcZ21U/R+4qJFV0+weygC4LkUAUvXGmOEO30KURUe8
        kP9l2xGe1RBz/AAQ==
Received: from ds.suse.cz (ds.suse.cz [10.100.12.205])
        by relay2.suse.de (Postfix) with ESMTP id 4426AA3BB4;
        Thu,  2 Sep 2021 15:28:17 +0000 (UTC)
Received: by ds.suse.cz (Postfix, from userid 10065)
        id 28C40DA72B; Thu,  2 Sep 2021 17:28:16 +0200 (CEST)
Date:   Thu, 2 Sep 2021 17:28:16 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Anand Jain <anand.jain@oracle.com>
Cc:     linux-btrfs@vger.kernel.org, dsterba@suse.com, l@damenly.su
Subject: Re: [PATCH V5 1/2] btrfs: fix lockdep warning while mounting sprout
 fs
Message-ID: <20210902152816.GX3379@twin.jikos.cz>
Reply-To: dsterba@suse.cz
Mail-Followup-To: dsterba@suse.cz, Anand Jain <anand.jain@oracle.com>,
        linux-btrfs@vger.kernel.org, dsterba@suse.com, l@damenly.su
References: <cover.1630370459.git.anand.jain@oracle.com>
 <215cb0c88d2b84557f8ec27e3f03c1c188df2935.1630370459.git.anand.jain@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <215cb0c88d2b84557f8ec27e3f03c1c188df2935.1630370459.git.anand.jain@oracle.com>
User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12)
Precedence: bulk
List-ID: <linux-btrfs.vger.kernel.org>
X-Mailing-List: linux-btrfs@vger.kernel.org

On Tue, Aug 31, 2021 at 09:21:28AM +0800, Anand Jain wrote:
> Following test case reproduces lockdep warning.
> 
>  Test case:
> 
>  $ mkfs.btrfs -f <dev1>
>  $ btrfstune -S 1 <dev1>
>  $ mount <dev1> <mnt>
>  $ btrfs device add <dev2> <mnt> -f
>  $ umount <mnt>
>  $ mount <dev2> <mnt>
>  $ umount <mnt>
> 
> The warning claims a possible ABBA deadlock between the threads initiated by
> [#1] btrfs device add and [#0] the mount.
> 
> ======================================================
> [ 540.743122] WARNING: possible circular locking dependency detected
> [ 540.743129] 5.11.0-rc7+ #5 Not tainted
> [ 540.743135] ------------------------------------------------------
> [ 540.743142] mount/2515 is trying to acquire lock:
> [ 540.743149] ffffa0c5544c2ce0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: clone_fs_devices+0x6d/0x210 [btrfs]
> [ 540.743458] but task is already holding lock:
> [ 540.743461] ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.743541] which lock already depends on the new lock.
> [ 540.743543] the existing dependency chain (in reverse order) is:
> 
> [ 540.743546] -> #1 (btrfs-chunk-00){++++}-{4:4}:
> [ 540.743566] down_read_nested+0x48/0x2b0
> [ 540.743585] __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.743650] btrfs_read_lock_root_node+0x70/0x200 [btrfs]
> [ 540.743733] btrfs_search_slot+0x6c6/0xe00 [btrfs]
> [ 540.743785] btrfs_update_device+0x83/0x260 [btrfs]
> [ 540.743849] btrfs_finish_chunk_alloc+0x13f/0x660 [btrfs] <--- device_list_mutex
> [ 540.743911] btrfs_create_pending_block_groups+0x18d/0x3f0 [btrfs]
> [ 540.743982] btrfs_commit_transaction+0x86/0x1260 [btrfs]
> [ 540.744037] btrfs_init_new_device+0x1600/0x1dd0 [btrfs]
> [ 540.744101] btrfs_ioctl+0x1c77/0x24c0 [btrfs]
> [ 540.744166] __x64_sys_ioctl+0xe4/0x140
> [ 540.744170] do_syscall_64+0x4b/0x80
> [ 540.744174] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> [ 540.744180] -> #0 (&fs_devs->device_list_mutex){+.+.}-{4:4}:
> [ 540.744184] __lock_acquire+0x155f/0x2360
> [ 540.744188] lock_acquire+0x10b/0x5c0
> [ 540.744190] __mutex_lock+0xb1/0xf80
> [ 540.744193] mutex_lock_nested+0x27/0x30
> [ 540.744196] clone_fs_devices+0x6d/0x210 [btrfs]
> [ 540.744270] btrfs_read_chunk_tree+0x3c7/0xbb0 [btrfs]
> [ 540.744336] open_ctree+0xf6e/0x2074 [btrfs]
> [ 540.744406] btrfs_mount_root.cold.72+0x16/0x127 [btrfs]
> [ 540.744472] legacy_get_tree+0x38/0x90
> [ 540.744475] vfs_get_tree+0x30/0x140
> [ 540.744478] fc_mount+0x16/0x60
> [ 540.744482] vfs_kern_mount+0x91/0x100
> [ 540.744484] btrfs_mount+0x1e6/0x670 [btrfs]
> [ 540.744536] legacy_get_tree+0x38/0x90
> [ 540.744537] vfs_get_tree+0x30/0x140
> [ 540.744539] path_mount+0x8d8/0x1070
> [ 540.744541] do_mount+0x8d/0xc0
> [ 540.744543] __x64_sys_mount+0x125/0x160
> [ 540.744545] do_syscall_64+0x4b/0x80
> [ 540.744547] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> [ 540.744551] other info that might help us debug this:
> [ 540.744552] Possible unsafe locking scenario:
> 
> [ 540.744553] CPU0 				CPU1
> [ 540.744554] ---- 				----
> [ 540.744555] lock(btrfs-chunk-00);
> [ 540.744557] 					lock(&fs_devs->device_list_mutex);
> [ 540.744560] 					lock(btrfs-chunk-00);
> [ 540.744562] lock(&fs_devs->device_list_mutex);
> [ 540.744564]
>  *** DEADLOCK ***
> 
> [ 540.744565] 3 locks held by mount/2515:
> [ 540.744567] #0: ffffa0c56bf7a0e0 (&type->s_umount_key#42/1){+.+.}-{4:4}, at: alloc_super.isra.16+0xdf/0x450
> [ 540.744574] #1: ffffffffc05a9628 (uuid_mutex){+.+.}-{4:4}, at: btrfs_read_chunk_tree+0x63/0xbb0 [btrfs]
> [ 540.744640] #2: ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.744708]
>  stack backtrace:
> [ 540.744712] CPU: 2 PID: 2515 Comm: mount Not tainted 5.11.0-rc7+ #5
> 
> But the device_list_mutex in clone_fs_devices() is redundant, as explained
> below.
> Two threads [1]  and [2] (below) could lead to clone_fs_device().
> 
> [1]
> open_ctree <== mount sprout fs
>  btrfs_read_chunk_tree()
>   mutex_lock(&uuid_mutex) <== global lock
>   read_one_dev()
>    open_seed_devices()
>     clone_fs_devices() <== seed fs_devices
>      mutex_lock(&orig->device_list_mutex) <== seed fs_devices
> 
> [2]
> btrfs_init_new_device() <== sprouting
>  mutex_lock(&uuid_mutex); <== global lock
>  btrfs_prepare_sprout()
>    lockdep_assert_held(&uuid_mutex)
>    clone_fs_devices(seed_fs_device) <== seed fs_devices
> 
> Both of these threads hold uuid_mutex which is sufficient to protect
> getting the seed device(s) freed while we are trying to clone it for
> sprouting [2] or mounting a sprout [1] (as above). A mounted seed
> device can not free/write/replace because it is read-only. An unmounted
> seed device can free by btrfs_free_stale_devices(), but it needs uuid_mutex.
> So this patch removes the unnecessary device_list_mutex in clone_fs_devices().
> And adds a lockdep_assert_held(&uuid_mutex) in clone_fs_devices().
> 
> Reported-by: Su Yue <l@damenly.su>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>

For the record, this patch has been added to misc-next.