mod_delayed_work() explosion due to 874bbfe6

[Mike Galbraith]

Hi Tejun,

I'm looking at a crash analysis that fingers 874bbfe6 as being the
culprit.  (I didn't do the analysis, but it appears to be correct)

Scenario: CPU168 calls mod_delayed_work(), is taken offline before the
timer expires.  Due to 874bbfe6, dwork->cpu is the now offline CPU168
vs the previous WORK_CPU_UNBOUND, timer fires on CPU131, it tries to
__queue_work() with cpu == the now offline CPU168, gets to...

        } else
                pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));

... and goes boom.

<snippet>
crash> p numa_node | grep 168
  [168]: ffff8c03fdf0e328
crash> rd ffff8c03fdf0e328
ffff8c03fdf0e328:  00000000ffffffff                    ........

Thus, pwq becomes 000000000. Then, as the result of reference to
pwq->pool, NULL reference occurs at (*PANIC).
</snippet>

What if anything is supposed to prevent this?

	-Mike