Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI

[Peter Zijlstra]

> Note the current ordering:
> 
>     cmpxchg(&qsd->pending, 0, 1)       get ipi
>     csd_lock(qsd->csd)                 xchg(&qsd->pending, 1)
>     send ipi                           csd_unlock(qsd->csd)
> 
> 
> So there shouldn't be racing updaters. Also ipi sender shouldn't
> race with ipi receiver, the update shouldn't always eventually see
> the unlock happening.

Yeah, I've not spotted how this particular train wreck happens either.

The problem is reproduction, it took me 9 hours to confirm I could
reproduce the problem on my machine. So how long to I run it with this
patch reverted to show its gone..
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