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

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Date:	Mon, 23 Sep 2013 08:00:06 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	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 Sun, 2013-09-22 at 10:47 -0700, H. Peter Anvin wrote:
> On 09/22/2013 09:24 AM, Peter Zijlstra wrote:
> > 
> > As to the problem of GCC moving r13 about, some archs have some
> > exceptions in the register allocator and leave some registers alone.
> > IIRC MIPS has this and uses one of those (istr there's 2) for the
> > per cpu base address.
> > 
> 
> You can force gcc to leave a register alone with the command line option
> -ffixed-r13.

Haven't tried that in a while but iirc, from the discussions I had with
the gcc folks, it didn't work the way we wanted.

Basically, it still assumes that it's not going to change at random
points, I can't have something like

register volatile unsigned long pre_cpu_offset asm("r13")

It will barf on the "volatile" and if I don't have it, it will make
assumptions that r13 doesn't change, and thus might copy its value
in another cpu accross preempt_enable/disable.

I think I need another sessions with gcc folks on that one.

Cheers,
Ben.


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