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    Subject[RFC][PATCH 3/3] configfs: Silence lockdep when destroying default groups
    When destroying a config_group having default groups, lockdep raises a warning
    because multiple locks of class I_MUTEX_NORMAL are taken in
    configfs_detach_prep() (the first one being in vfs_rmdir()). However this
    recursive locking is another instance of I_MUTEX_PARENT -> I_MUTEX_CHILD
    dependency, which was already checked correct when creating the config_group,
    and which lockdep cannot handle cleanly because of the variable depth.

    This patch silences lockdep when destroying default groups by simply hiding the
    inode mutex locks from lockdep in configfs_detach_prep(),
    configfs_detach_rollback(), and detach_groups().

    =============================================
    [ INFO: possible recursive locking detected ]
    2.6.26-rc3 #5
    ---------------------------------------------
    rmdir/1499 is trying to acquire lock:
    (&sb->s_type->i_mutex_key#11){--..}, at: [<ffffffff802d2529>] configfs_detach_prep+0x54/0x89

    but task is already holding lock:
    (&sb->s_type->i_mutex_key#11){--..}, at: [<ffffffff80296513>] vfs_rmdir+0x49/0xac

    other info that might help us debug this:
    2 locks held by rmdir/1499:
    #0: (&sb->s_type->i_mutex_key#3/1){--..}, at: [<ffffffff80298138>] do_rmdir+0x82/0x108
    #1: (&sb->s_type->i_mutex_key#11){--..}, at: [<ffffffff80296513>] vfs_rmdir+0x49/0xac

    stack backtrace:
    Pid: 1499, comm: rmdir Not tainted 2.6.26-rc3 #5

    Call Trace:
    [<ffffffff8024afe9>] __lock_acquire+0x8d2/0xc78
    [<ffffffff80266399>] filemap_fault+0x1cf/0x332
    [<ffffffff802d2529>] configfs_detach_prep+0x54/0x89
    [<ffffffff8024b762>] lock_acquire+0x51/0x6c
    [<ffffffff802d2529>] configfs_detach_prep+0x54/0x89
    [<ffffffff80248331>] debug_mutex_lock_common+0x16/0x23
    [<ffffffff805d6244>] mutex_lock_nested+0xcd/0x23b
    [<ffffffff802d2529>] configfs_detach_prep+0x54/0x89
    [<ffffffff802d3656>] configfs_rmdir+0xb8/0x1c3
    [<ffffffff80296535>] vfs_rmdir+0x6b/0xac
    [<ffffffff8029816d>] do_rmdir+0xb7/0x108
    [<ffffffff8024a2a2>] trace_hardirqs_on+0xef/0x113
    [<ffffffff805d7364>] trace_hardirqs_on_thunk+0x35/0x3a
    [<ffffffff8020b0cb>] system_call_after_swapgs+0x7b/0x80


    Signed-off-by: Louis Rilling <Louis.Rilling@kerlabs.com>
    ---
    fs/configfs/dir.c | 18 +++++++++++++++++-
    1 file changed, 17 insertions(+), 1 deletion(-)


    Index: b/fs/configfs/dir.c
    ===================================================================
    --- a/fs/configfs/dir.c 2008-05-20 18:19:41.000000000 +0200
    +++ b/fs/configfs/dir.c 2008-05-20 18:19:45.000000000 +0200
    @@ -352,7 +352,12 @@ static int configfs_detach_prep(struct d
    if (sd->s_type & CONFIGFS_NOT_PINNED)
    continue;
    if (sd->s_type & CONFIGFS_USET_DEFAULT) {
    + /* Hide this mutex from lockdep for the same reasons as in
    + * populate_groups()
    + */
    + lockdep_off();
    mutex_lock(&sd->s_dentry->d_inode->i_mutex);
    + lockdep_on();
    /* Mark that we've taken i_mutex */
    sd->s_type |= CONFIGFS_USET_DROPPING;

    @@ -388,7 +393,12 @@ static void configfs_detach_rollback(str

    if (sd->s_type & CONFIGFS_USET_DROPPING) {
    sd->s_type &= ~CONFIGFS_USET_DROPPING;
    + /* This mutex was hidden from lockdep in
    + * configfs_detach_prep()
    + */
    + lockdep_off();
    mutex_unlock(&sd->s_dentry->d_inode->i_mutex);
    + lockdep_on();
    }
    }
    }
    @@ -475,8 +485,14 @@ static void detach_groups(struct config_
    * has taken our i_mutex for us. Drop it.
    * From mkdir/register cleanup, there is no sem held.
    */
    - if (sd->s_type & CONFIGFS_USET_DROPPING)
    + if (sd->s_type & CONFIGFS_USET_DROPPING) {
    + /* This mutex was hidden from lockdep in
    + * configfs_detach_prep()
    + */
    + lockdep_off();
    mutex_unlock(&child->d_inode->i_mutex);
    + lockdep_on();
    + }

    d_delete(child);
    dput(child);
    --
    Dr Louis Rilling Kerlabs
    Skype: louis.rilling Batiment Germanium
    Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
    http://www.kerlabs.com/ 35700 Rennes


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