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

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Date:	Fri, 20 Sep 2013 13:37:33 -0500
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@....ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.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, Sep 20, 2013 at 07:30:14PM +0200, Thomas Gleixner wrote:
> On Fri, 20 Sep 2013, Frederic Weisbecker wrote:
> 
> > Now just for clarity, what do we then do with inline sofirq
> > executions: on local_bh_enable() for example, or explicit calls to
> > do_softirq() other than irq exit?  Should we keep the current switch
> > to a different softirq stack? If we have a generic irq stack (used
> > for both hard and soft) that is big enough, perhaps we can also
> > switch to this generic irq stack for inline softirqs executions?
> > After all there is no much point in keeping a separate stack for
> > that: this result in cache misses if the inline softirq is
> > interrupted by a hardirq, also inlined softirqs can't happen in
> > hardirq, so there should be no much risk of overruns.
> 
> We can use the same irqstack for this because from the irqstack point
> of view, thats the same as if softirqs get executed from
> irq_exit().

Ok, so I see that's what x86-64 is doing. But x86-32 seems to be using different
stacks for hard and soft irqs for no much reasons (expept maybe to avoid overrun if
the hardirq). And x86-32 only switches to hardirq
stack for the handler. So probably we can use the same stack for the whole and extend
it to before irq_enter() and after irq_exit(), like you suggested.

BTW, we still want the 1st patch of my series I think, as it simply consolidate existing code.
Using the same stack for hard and soft irqs is independant from that.

If you're ok with it, would you agree to apply it?

Thanks.
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