Messages in this thread | | | Subject | Re: preempt-related hangs | From | Robert Love <> | Date | 24 Mar 2002 21:30:26 -0500 |
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On Sun, 2002-03-24 at 21:11, Andrew Morton wrote:
> OK, this patch fixed it. I don't know why.
Eh, odd. That effectively disables kernel preemption around set_cpus_allowed, but preemption is already disabled by the task_rq_lock call. Note, however, preemption is enabled by the task_rq_unlock and thus wake_up_process is called with preemption enabled.
With your patch, preemption is now disabled across the call, and subsequently the task_rq_unlock in try_to_wake_up will never call preempt_schedule and your lock does not happen.
The actual problem may be elsewhere, and this just hides it. This is pretty clear, since we would get a similar effect just wrapping wake_up_process in preempt_disable. But, oh, try_to_wake_up disables preempt, too ... hrm.
Hm, what if try_to_wake_up wakes up a process and then preemptively schedules into it and it wants to acquire the req.sem semaphore, but cannot, as it is still taken by set_cpus_allowed? The semaphore seems to just be used in the migration code.
So we have init spinning on softirq threads to come up and then we have a deadlock on req.sem from set_cpus_allowed and into the migration thread?
Bleh ... Ingo?
Robert Love
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