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    Subject[PATCH 4.9 03/42] btrfs: fix lockdep splat when reading qgroup config on mount
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 3d05cad3c357a2b749912914356072b38435edfa upstream.

    Lockdep reported the following splat when running test btrfs/190 from
    fstests:

    [ 9482.126098] ======================================================
    [ 9482.126184] WARNING: possible circular locking dependency detected
    [ 9482.126281] 5.10.0-rc4-btrfs-next-73 #1 Not tainted
    [ 9482.126365] ------------------------------------------------------
    [ 9482.126456] mount/24187 is trying to acquire lock:
    [ 9482.126534] ffffa0c869a7dac0 (&fs_info->qgroup_rescan_lock){+.+.}-{3:3}, at: qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.126647]
    but task is already holding lock:
    [ 9482.126777] ffffa0c892ebd3a0 (btrfs-quota-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x120 [btrfs]
    [ 9482.126886]
    which lock already depends on the new lock.

    [ 9482.127078]
    the existing dependency chain (in reverse order) is:
    [ 9482.127213]
    -> #1 (btrfs-quota-00){++++}-{3:3}:
    [ 9482.127366] lock_acquire+0xd8/0x490
    [ 9482.127436] down_read_nested+0x45/0x220
    [ 9482.127528] __btrfs_tree_read_lock+0x27/0x120 [btrfs]
    [ 9482.127613] btrfs_read_lock_root_node+0x41/0x130 [btrfs]
    [ 9482.127702] btrfs_search_slot+0x514/0xc30 [btrfs]
    [ 9482.127788] update_qgroup_status_item+0x72/0x140 [btrfs]
    [ 9482.127877] btrfs_qgroup_rescan_worker+0xde/0x680 [btrfs]
    [ 9482.127964] btrfs_work_helper+0xf1/0x600 [btrfs]
    [ 9482.128039] process_one_work+0x24e/0x5e0
    [ 9482.128110] worker_thread+0x50/0x3b0
    [ 9482.128181] kthread+0x153/0x170
    [ 9482.128256] ret_from_fork+0x22/0x30
    [ 9482.128327]
    -> #0 (&fs_info->qgroup_rescan_lock){+.+.}-{3:3}:
    [ 9482.128464] check_prev_add+0x91/0xc60
    [ 9482.128551] __lock_acquire+0x1740/0x3110
    [ 9482.128623] lock_acquire+0xd8/0x490
    [ 9482.130029] __mutex_lock+0xa3/0xb30
    [ 9482.130590] qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.131577] btrfs_read_qgroup_config+0x43a/0x550 [btrfs]
    [ 9482.132175] open_ctree+0x1228/0x18a0 [btrfs]
    [ 9482.132756] btrfs_mount_root.cold+0x13/0xed [btrfs]
    [ 9482.133325] legacy_get_tree+0x30/0x60
    [ 9482.133866] vfs_get_tree+0x28/0xe0
    [ 9482.134392] fc_mount+0xe/0x40
    [ 9482.134908] vfs_kern_mount.part.0+0x71/0x90
    [ 9482.135428] btrfs_mount+0x13b/0x3e0 [btrfs]
    [ 9482.135942] legacy_get_tree+0x30/0x60
    [ 9482.136444] vfs_get_tree+0x28/0xe0
    [ 9482.136949] path_mount+0x2d7/0xa70
    [ 9482.137438] do_mount+0x75/0x90
    [ 9482.137923] __x64_sys_mount+0x8e/0xd0
    [ 9482.138400] do_syscall_64+0x33/0x80
    [ 9482.138873] entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [ 9482.139346]
    other info that might help us debug this:

    [ 9482.140735] Possible unsafe locking scenario:

    [ 9482.141594] CPU0 CPU1
    [ 9482.142011] ---- ----
    [ 9482.142411] lock(btrfs-quota-00);
    [ 9482.142806] lock(&fs_info->qgroup_rescan_lock);
    [ 9482.143216] lock(btrfs-quota-00);
    [ 9482.143629] lock(&fs_info->qgroup_rescan_lock);
    [ 9482.144056]
    *** DEADLOCK ***

    [ 9482.145242] 2 locks held by mount/24187:
    [ 9482.145637] #0: ffffa0c8411c40e8 (&type->s_umount_key#44/1){+.+.}-{3:3}, at: alloc_super+0xb9/0x400
    [ 9482.146061] #1: ffffa0c892ebd3a0 (btrfs-quota-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x120 [btrfs]
    [ 9482.146509]
    stack backtrace:
    [ 9482.147350] CPU: 1 PID: 24187 Comm: mount Not tainted 5.10.0-rc4-btrfs-next-73 #1
    [ 9482.147788] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
    [ 9482.148709] Call Trace:
    [ 9482.149169] dump_stack+0x8d/0xb5
    [ 9482.149628] check_noncircular+0xff/0x110
    [ 9482.150090] check_prev_add+0x91/0xc60
    [ 9482.150561] ? kvm_clock_read+0x14/0x30
    [ 9482.151017] ? kvm_sched_clock_read+0x5/0x10
    [ 9482.151470] __lock_acquire+0x1740/0x3110
    [ 9482.151941] ? __btrfs_tree_read_lock+0x27/0x120 [btrfs]
    [ 9482.152402] lock_acquire+0xd8/0x490
    [ 9482.152887] ? qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.153354] __mutex_lock+0xa3/0xb30
    [ 9482.153826] ? qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.154301] ? qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.154768] ? qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.155226] qgroup_rescan_init+0x43/0xf0 [btrfs]
    [ 9482.155690] btrfs_read_qgroup_config+0x43a/0x550 [btrfs]
    [ 9482.156160] open_ctree+0x1228/0x18a0 [btrfs]
    [ 9482.156643] btrfs_mount_root.cold+0x13/0xed [btrfs]
    [ 9482.157108] ? rcu_read_lock_sched_held+0x5d/0x90
    [ 9482.157567] ? kfree+0x31f/0x3e0
    [ 9482.158030] legacy_get_tree+0x30/0x60
    [ 9482.158489] vfs_get_tree+0x28/0xe0
    [ 9482.158947] fc_mount+0xe/0x40
    [ 9482.159403] vfs_kern_mount.part.0+0x71/0x90
    [ 9482.159875] btrfs_mount+0x13b/0x3e0 [btrfs]
    [ 9482.160335] ? rcu_read_lock_sched_held+0x5d/0x90
    [ 9482.160805] ? kfree+0x31f/0x3e0
    [ 9482.161260] ? legacy_get_tree+0x30/0x60
    [ 9482.161714] legacy_get_tree+0x30/0x60
    [ 9482.162166] vfs_get_tree+0x28/0xe0
    [ 9482.162616] path_mount+0x2d7/0xa70
    [ 9482.163070] do_mount+0x75/0x90
    [ 9482.163525] __x64_sys_mount+0x8e/0xd0
    [ 9482.163986] do_syscall_64+0x33/0x80
    [ 9482.164437] entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [ 9482.164902] RIP: 0033:0x7f51e907caaa

    This happens because at btrfs_read_qgroup_config() we can call
    qgroup_rescan_init() while holding a read lock on a quota btree leaf,
    acquired by the previous call to btrfs_search_slot_for_read(), and
    qgroup_rescan_init() acquires the mutex qgroup_rescan_lock.

    A qgroup rescan worker does the opposite: it acquires the mutex
    qgroup_rescan_lock, at btrfs_qgroup_rescan_worker(), and then tries to
    update the qgroup status item in the quota btree through the call to
    update_qgroup_status_item(). This inversion of locking order
    between the qgroup_rescan_lock mutex and quota btree locks causes the
    splat.

    Fix this simply by releasing and freeing the path before calling
    qgroup_rescan_init() at btrfs_read_qgroup_config().

    CC: stable@vger.kernel.org # 4.4+
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Reviewed-by: David Sterba <dsterba@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/qgroup.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/btrfs/qgroup.c
    +++ b/fs/btrfs/qgroup.c
    @@ -461,13 +461,13 @@ next2:
    break;
    }
    out:
    + btrfs_free_path(path);
    fs_info->qgroup_flags |= flags;
    if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
    clear_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
    else if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN &&
    ret >= 0)
    ret = qgroup_rescan_init(fs_info, rescan_progress, 0);
    - btrfs_free_path(path);

    if (ret < 0) {
    ulist_free(fs_info->qgroup_ulist);

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