linux-kernel - Re: [patch 01/20] idle: Move x86ism out of generic code

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Date:	Mon, 29 Feb 2016 19:48:10 +0000
From:	Will Deacon <will.deacon@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Brian Gerst <brgerst@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Peter Anvin <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	linux-arch@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa@....edu>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [patch 01/20] idle: Move x86ism out of generic code

On Mon, Feb 29, 2016 at 08:35:41PM +0100, Thomas Gleixner wrote:
> On Sat, 27 Feb 2016, Brian Gerst wrote:
> > >         arch_cpu_idle_prepare();
> > >         cpu_idle_loop();
> > >  }
> > 
> > Does this actually work with stack protector enabled?
> > boot_init_stack_canary() is inlined while arch_cpu_idle_prepare() is
> > not.
> 
> Stupid me. No it does of course not. I could have sworn that I tested that,
> but obvioulsy not.
> 
> I drop that patch, but actually the real question is whether we can drop that
> '#ifdef x86' around that boot_init_stack_canary() invocation.
> 
> AFAICT, neither arm, arm64 nor mips and sh call it on anything else than the
> boot cpu. I can't see why that would be an issue on those architectures and
> why it would be a problem if the boot cpu calls it again here.
> 
> CC'ed the relevant maintainers. Is there any issue with the patch below?

On arm[64], the canary is unfortunately global, so I don't think it would
be safe to update it live like this without effectively stopping the
machine and forcing everybody into idle.

Will