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    SubjectRe: [Patch] workqueue: move lockdep annotations up to destroy_workqueue()
    Hello, guys.

    On 04/01/2010 11:45 AM, Cong Wang wrote:
    >> OK, but nobody should take cpu_maps_update_begin() under wq->lockdep_map,
    >> in particular work->func() must not.
    >>
    >> I must have missed something, but it seems to me this patch tries to
    >> supress the valid warning.
    >>
    >> Could you please clarify?
    >
    > Sure, below is the whole warning. Please teach me how this is valid.

    I still have some trouble interpreting lockdep warnings. Please
    correct me if I get something wrong.

    > modprobe/5264 is trying to acquire lock:
    > ((bond_dev->name)){+.+...}, at: [<ffffffff8108524a>] cleanup_workqueue_thread+0x2b/0x10b
    >
    > but task is already holding lock:
    > (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff810631d1>] cpu_maps_update_begin+0x1e/0x27

    This (cpu hotplug -> wq) is the expected sequence. Plug cpu
    hotplugging and then flush cpu workqueues.

    > which lock already depends on the new lock.

    But lockdep says the other way around has already happened.

    > the existing dependency chain (in reverse order) is:
    >
    > -> #3 (cpu_add_remove_lock){+.+.+.}:
    > [<ffffffff810a6bc1>] validate_chain+0x1019/0x1540
    > [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
    > [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
    > [<ffffffff815523f8>] mutex_lock_nested+0x64/0x4e9
    > [<ffffffff810631d1>] cpu_maps_update_begin+0x1e/0x27
    > [<ffffffff810853cd>] destroy_workqueue+0x41/0x107
    > [<ffffffffa0839d32>] bond_uninit+0x524/0x58a [bonding]
    > [<ffffffff8146967b>] rollback_registered_many+0x205/0x2e3
    > [<ffffffff81469783>] unregister_netdevice_many+0x2a/0x75
    > [<ffffffff8147ada3>] __rtnl_kill_links+0x8b/0x9d
    > [<ffffffff8147adea>] __rtnl_link_unregister+0x35/0x72
    > [<ffffffff8147b293>] rtnl_link_unregister+0x2c/0x43
    > [<ffffffffa0845ca6>] bonding_exit+0x5a/0x76 [bonding]
    > [<ffffffff810b7749>] sys_delete_module+0x306/0x3b1
    > [<ffffffff81003a5b>] system_call_fastpath+0x16/0x1b

    This is bond_uninit() calling destroy_workqueue() but I don't get how
    this thread would be already holding wq lock.

    > -> #2 (rtnl_mutex){+.+.+.}:
    > [<ffffffff810a6bc1>] validate_chain+0x1019/0x1540
    > [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
    > [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
    > [<ffffffff815523f8>] mutex_lock_nested+0x64/0x4e9
    > [<ffffffff8147af16>] rtnl_lock+0x1e/0x27
    > [<ffffffffa0836779>] bond_mii_monitor+0x39f/0x74b [bonding]
    > [<ffffffff8108654f>] worker_thread+0x2da/0x46c
    > [<ffffffff8108b1ea>] kthread+0xdd/0xec
    > [<ffffffff81004894>] kernel_thread_helper+0x4/0x10
    >
    > -> #1 ((&(&bond->mii_work)->work)){+.+...}:
    > [<ffffffff810a6bc1>] validate_chain+0x1019/0x1540
    > [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
    > [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
    > [<ffffffff81086542>] worker_thread+0x2cd/0x46c
    > [<ffffffff8108b1ea>] kthread+0xdd/0xec
    > [<ffffffff81004894>] kernel_thread_helper+0x4/0x10

    These two are form a workqueue worker thread and I don't quite
    understand why they are here.

    > -> #0 ((bond_dev->name)){+.+...}:
    > [<ffffffff810a6696>] validate_chain+0xaee/0x1540
    > [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
    > [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
    > [<ffffffff81085278>] cleanup_workqueue_thread+0x59/0x10b
    > [<ffffffff81085428>] destroy_workqueue+0x9c/0x107
    > [<ffffffffa0839d32>] bond_uninit+0x524/0x58a [bonding]
    > [<ffffffff8146967b>] rollback_registered_many+0x205/0x2e3
    > [<ffffffff81469783>] unregister_netdevice_many+0x2a/0x75
    > [<ffffffff8147ada3>] __rtnl_kill_links+0x8b/0x9d
    > [<ffffffff8147adea>] __rtnl_link_unregister+0x35/0x72
    > [<ffffffff8147b293>] rtnl_link_unregister+0x2c/0x43
    > [<ffffffffa0845ca6>] bonding_exit+0x5a/0x76 [bonding]
    > [<ffffffff810b7749>] sys_delete_module+0x306/0x3b1
    > [<ffffffff81003a5b>] system_call_fastpath+0x16/0x1b

    This seems to be from the original thread of frame#3. It's grabbing
    wq lock here but the problem is that the lock will be released
    immediately, so bond_dev->name (the wq) can't be held by the time it
    reaches frame#3. How is this dependency chain completed? Is it
    somehow transitive through rtnl_mutex?

    > other info that might help us debug this:
    >
    > 2 locks held by modprobe/5264:
    > #0: (rtnl_mutex){+.+.+.}, at: [<ffffffff8147af16>] rtnl_lock+0x1e/0x27
    > #1: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff810631d1>] cpu_maps_update_begin+0x1e/0x27

    Isn't there a circular dependency here? bonding_exit() calls
    destroy_workqueue() under rtnl_mutex but destroy_workqueue() should
    flush works which could be trying to grab rtnl_lock. Or am I
    completely misunderstanding locking here?

    Thanks.

    --
    tejun


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