Messages in this thread | | | From | Neil Brown <> | Date | Wed, 11 Oct 2006 23:08:21 +1000 | Subject | _cpu_down deadlock [was Re: 2.6.19-rc1-mm1] |
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On Wednesday October 11, arjan@infradead.org wrote: > > > > blocking_notifier_call_chain is > > > down_read(&nh->rwsem); > > > ret = notifier_call_chain(&nh->head, val, v); > > > up_read(&nh->rwsem); > > > > > > and so holds ->rwsem while calling the callback. > > > So the locking sequence ends up as: > > > > > > down_read(&cpu_chain.rwsem); > > > mutex_lock(&workqueue_mutex); > > > up_read(&cpu_chain.rwsem); > > > > > > down_read(&cpu_chain.rwsem); > > > mutex_unlock(&workqueue_mutex); > > > up_read(&workqueue_mutex); > > > > > > and lockdep doesn't seem to like this. It sees workqueue_mutex > > > claimed while cpu_chain.rwsem is held. and then it sees > > > cpu_chain.rwsem claimed while workqueue_mutex is held, which looks a > > > bit like a class ABBA deadlock. > > > Of course because it is a 'down_read' rather than a 'down', it isn't > > > really a dead lock. > > ok can you explain to me why "down_read" doesn't make this a deadlock > while "down" would make it a deadlock? I have trouble following your > reasoning..... > > (remember that rwsems are strictly fair)
I see your point.
While thread A holds just workqueue_mutex, thread B takes cpu_chain.rwsem for read then tries to take workqueue_mutex and blocks. Now thread C tries to get a write lock on cpu_chain.rwsem and blocks as well. Finally thread A moves on to try to get a read lock on cpu_chain.rwsem and this blocks because thread C is waiting for a write lock.
So A waits on B and C, C waits on B, B waits on A. Deadlock.
I guess _cpu_down should down_read(&cpu_chain.rwsem); and then call notifier_call_chain multiple times. I wonder if that would be safe.
Who do we blame this on? Are you still the cpu-hot-plug guy Rusty?
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