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

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Date:	Tue, 24 Sep 2013 06:57:17 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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 Mon, 2013-09-23 at 13:59 -0400, Chris Metcalf wrote:
> We just came up against this on tilegx with a customer bug report in a
> PREEMPT environment.  On tile, %tp is a GPR that points to the percpu area.
> The following seems to be the right abstraction -- though I'd also argue
> that letting barrier() clobber not just memory, but %tp, might be a better
> solution, but it's not clear what the best way is to do per-architecture
> overrides of per-compiler definitions like barrier().  See also the ARM v7
> code, which has to do something similar, though their percpu pointer is
> not a GPR, which changes the tradeoffs somewhat.

Hrm, if I read correctly what you did is that you read "tp" into another
register *and* also mark that action as clobbering the top int on the stack ?

I don't quite get what the stack clobber brings you here and how it works
around the fact that gcc might still cache that copy of tp into another
register accross preempt_enable/disable...

It's hard to tell with gcc ... the best I've had so far as an option was
something that would mark my per-cpu register (r13) *itself* as clobbered...

Cheers,
Ben.


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