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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

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 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 ?

> >  - 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.

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

Cheers,
Ben.