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

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Message-ID: <1380056147.5443.61.camel@pasglop>
Date:	Wed, 25 Sep 2013 06:55:47 +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 Tue, 2013-09-24 at 15:56 +0200, Frederic Weisbecker wrote:
> On Tue, Sep 24, 2013 at 02:42:57PM +1000, Benjamin Herrenschmidt wrote:
> > On Tue, 2013-09-24 at 04:44 +0200, Frederic Weisbecker wrote:
> > > So the safest way to fix this is to unconditionally call do_softirq()
> > > from irq_exit().
> > > A performance penalty may come along but safety primes.
> > > 
> > > We should probably do that and work on longer term solutions (Kconfig
> > > based arch switch, etc...)
> > > for the next merge window?
> > 
> > As you prefer, though I'm keen on getting the "fast" version in RHEL7 if
> > RH will take it :-)
> 
> So what is the fast version? Converting __do_softirq() to do_softirq()
> unconditionally.
> 
> RH will accept any fix that goes upstream.

No, me fixing powerpc do_IRQ to do irq_exit run on the irq stack, and
your fix for everybody else with an ifdef such that x86_64 and powerpc
get to skip the additional stack switch.

> > 
> > From the generic code POV, it's a one-liner #ifdef to select between
> > do_softirq and __do_softirq() right ? Then it's up to the arch to
> > #define I_CAN_DO_FAST !
> 
> I'd rather say #define I_CAN_DO_SAFE :)
> 
> But I guess the kind of symbol we want is some ARCH_HAS_IRQ_STACK_LOW_HANDLER

ARCH_IRQ_EXIT_ON_IRQ_STACK

Cheers,
Ben.

> > 
> > > I'll respin the series plus the regression fix, unless somebody has a
> > > better solution.
> > 
> > Cheers,
> > Ben.
> > 
> > 


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