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

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Date:	Sun, 22 Sep 2013 14:39:14 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.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 Sat, 2013-09-21 at 19:01 -0700, H. Peter Anvin wrote:
> On 09/21/2013 02:45 PM, Benjamin Herrenschmidt wrote:
> > 
> > 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.
> > 
> 
> For the record, I intend to remove thread_info from the stack on x86 and
> instead merge it with task_struct as a single structure pointed to with
> a percpu variable.

Last I looked, our per-cpu codegen was pretty poor... but then we have
this "PACA" (somewhat arch specific per-cpu blob that is separate from
the rest of per-cpu because of a mix of historical reasons and the fact
that it has to be allocated in a specific part of memory at boot time)
which we point to directly via a GPR, so we could point to it via PACA.

How do you do your per-cpu on x86 ? On powerpc we struggle because we
try to dedicate a register (r13) to this PACA (the per-cpu offset hangs
off it), but we constantly run into issues where gcc copies r13 to
another register and then indexes off that, even accross
preempt_enable/disable sections, or worst such as saving/restoring from
the stack. We can't seem to get the compiler to treat it appropriately
as volatile.

Ben.

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