oom-killer causes lockups in cpuset_excl_nodes_overlap()

The oom-killer causes lockups because it calls
cpuset_excl_nodes_overlap() with tasklist_lock read-locked.
cpuset_excl_nodes_overlap() gets cpuset_sem (or callback_sem in
later linux versions) semaphore, which might_sleep even if the
semaphore could be down without sleeping.  If processes call
exit() or fork() when the oom-killer sleeps in the down(), they
lockup because they call write_lock_irq(&tasklist_lock).

The lockup occurred on linux-2.6.14.  The problem also seems to exist
in linux-2.6.15-rc5-mm3 and linux-2.6.15-rc7.

Regards,

-- 
KUROSAWA, Takahiro
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