rcu_do_batch: rcu_data->qlen is not irq safe

[Oleg Nesterov <oleg@tv-sign.ru>]

rcu_do_batch() decrements rdp->qlen with irqs enabled.
This is not good, it can also be modified by call_rcu()
from interrupt.

So, is it worth fixing? The problem is mostly theoretical.

If yes, is it ok to use local_t ? Iirc, the were some
problems with local_t on some arches. Sometimes it is
just atomic_t ...

Otherwise, we can update ->qlen after the main loop,

	local_irq_disable();
	rdp->qlen -= count;
	local_irq_enable();

What dou you think?

Oleg.

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