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

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Date:	Sun, 22 Sep 2013 07:45:01 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.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 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.

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