> This is, AFAICT, overkill (the UP compilation fix appreciated though).
>
> When a new CPU comes up, there is a semaphore which is held through the
> notifier, so you can't have two CPUs come up at once.
>
> Therefore, the new migration thread is either started on a completely
> active cpu (ie. there's a migration thread on that CPU to use), or it's
> already on the new cpu, in which case set_cpus_allowed is a noop.
>
> What am I missing? [...]
you are missing the following situation: if a newly forked migration
thread is put on a non-boot CPU that is already started up. Because it's
the target CPU's migration thread that counts - and since the newly online
CPU doesnt have one, we could deadlock.
so current BK is the right behavior.
Ingo
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