Messages in this thread | | | Date | Thu, 01 Apr 2010 12:09:22 +0800 | From | Cong Wang <> | Subject | Re: [Patch] workqueue: move lockdep annotations up to destroy_workqueue() |
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Tejun Heo wrote: > 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.
> >> -> #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.
> >> 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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