Re: [RFC GIT PULL] softirq: Consolidation and stack overrun fix

[Frederic Weisbecker]

On Sun, Sep 22, 2013 at 07:45:01AM +1000, Benjamin Herrenschmidt wrote:
> On Sat, 2013-09-21 at 13:58 -0500, Frederic Weisbecker wrote:
> 
> > Now certainly what needs to be fixed then is archs that don't have
> > __ARCH_IRQ_EXIT_IRQS_DISABLED
> > or archs that have any other significant opportunity to nest interrupt.
> 
> Interesting. I notice we don't define it on powerpc

Yeah, x86 doesn't define it either. In fact few archs do.

> but we don't enable
> IRQs in do_IRQ either... our path is very similar to x86 in this regard,
> the only thing that can cause them to become enabled would be if a
> driver interrupt handler did local_irq_enable().
> 
> It used to be fairly common for drivers to do spin_unlock_irq() which
> would unconditionally re-enable. Did we add WARNs or lockdep logic to
> catch these nowadays ?

Right there is a check in handle_irq_event_percpu() that warns if the handler
exits with irqs enabled.

And irq_exit() also warns when (__ARCH_IRQ_EXIT_IRQS_DISABLED && !irq_disabled())

> 
> > >  - process context doing local_bh_enable, and a bh became pending
> > > while it was disabled. See above: this needs a stack switch. Which
> > > stack to use is open, again assuming that a hardirq coming in will
> > > switch to yet another stack.
> > 
> > Right. Now if we do like Thomas suggested, we can have a common irq
> > stack that is big enough for hard and softirqs. After all there should
> > never be more than two or three nesting irq contexts:
> > hardirq->softirq->hardirq, softirq->hardirq, ...
> > 
> > At least if we put aside the unsane archs that can nest irqs somehow.
> 
> I really don't like the "larger" irq stack ... probably because I can't
> make it work easily :-) See my previous comment about how we get to
> thread_info on ppc.
> 
> What I *can* do that would help I suppose would be to switch to the irq
> stack before irq_enter/exit which would at least mean that softirq would
> run from the top of the irq stack which is better than the current
> situation.

Yeah I think that doing this should solve the biggest part of the problem on ppc.
You'll at least ensure that you have splitup stacks for tasks and softirq/irq stacks.

> 
> I'm fact I'll whip up a quick fix see if that might be enough of a band
> aid for RHEL7.
> 
> Cheers,
> Ben.
> 
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