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SubjectRe: preempt-related hangs
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

Bleh ... Ingo?

Robert Love

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