linux-kernel - Re: [RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in delay

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Date:	Wed, 9 Mar 2016 19:10:03 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Huang Rui <ray.huang@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	spg_linux_kernel@....com, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in
 delay_mwaitx()

On Wed, Mar 09, 2016 at 09:56:39AM -0800, Andy Lutomirski wrote:
> On Mar 9, 2016 3:38 AM, "Borislav Petkov" <bp@...en8.de> wrote:
> >
> > From: Borislav Petkov <bp@...e.de>
> >
> > So Andy had a good idea about using a cacheline-aligned, seldomly used
> > per-cpu var as the MONITORX target but we can't use it in preemptible
> > context. The first simple idea I have is to disable preemption around us
> > dereffing it.
> 
> What's the actual problem?  Is it the preempt warnings and, if so,
> would raw_cpu_ptr fix it?

Yeah, it is the warning:

[    1.565876] BUG: using smp_processor_id() in preemptible [00000000] code: udevd/312
[    1.566123] caller is delay_mwaitx+0x40/0xa0

and yes, I think so. I don't think we care about being in preemptible
context since we're going idle anyway and doesn't matter which cpu_tss
we touch.

Yeah, I'll use raw_cpu_ptr...

> It may pay to move it into the loop, though.

... and won't need to do that.

Thanks for the idea.

-- 
Regards/Gruss,
    Boris.

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