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

[Benjamin Herrenschmidt]

On Fri, 2013-09-20 at 13:11 +0200, Peter Zijlstra wrote:
> On Fri, Sep 20, 2013 at 01:03:17PM +0200, Thomas Gleixner wrote:
> > On Thu, 19 Sep 2013, Linus Torvalds wrote:
> > 
> > > On Thu, Sep 19, 2013 at 2:51 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> > > >
> > > > It fixes stacks overruns reported by Benjamin Herrenschmidt:
> > > > http://lkml.kernel.org/r/1378330796.4321.50.camel%40pasglop
> > > 
> > > So I don't really dislike this patch-series, but isn't "irq_exit()"
> > > (which calls the new softirq_on_stack()) already running in the
> > > context of the irq stack? And it's run at the very end of the irq
> > > processing, so the irq stack should be empty too at that point.
> > 
> > Right, but most of the implementations are braindamaged.
> > 
> >       irq_enter();
> >       handle_irq_on_hardirq_stack();
> >       irq_exit();
> 
> I was only just staring at i386 and found it did exactly that. It had to
> jump through preempt_count hoops to make that work and obviously I
> hadn't test-build the preempt patches on i386.

Right and powerpc does the switch even later when calling the individual
handlers.

Ben.

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