Tejun Heo wrote:
quoted text
> 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.


destroy_workqueue() does hold wq lock and then releases it.

quoted text
> 
>> -> #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?

wq lock is held *after* cpu_add_remove_lock, lockdep also said this,
the process is trying to hold wq lock while having cpu_add_remove_lock.


quoted text
> 
>> 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?

Sure, that is why I sent another patch for bonding. :)

After this patch, another lockdep warning appears, it is exactly what you expect.

Thanks.
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