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

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Date:	Sat, 21 Sep 2013 10:55:42 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@....ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	James Hogan <james.hogan@...tec.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC GIT PULL] softirq: Consolidation and stack overrun fix

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@...il.com> 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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