linux-kernel - Re: [lkp] [x86/fpu] 58122bf1d8: WARNING: CPU: 0 PID: 1 at arch/x86/include/asm/fpu/internal.h:529 fpu_

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Date:	Sat, 27 Feb 2016 14:13:37 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	kernel test robot <ying.huang@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>, lkp@...org,
	LKML <linux-kernel@...r.kernel.org>,
	yu-cheng yu <yu-cheng.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [lkp] [x86/fpu] 58122bf1d8: WARNING: CPU: 0 PID: 1 at
 arch/x86/include/asm/fpu/internal.h:529 fpu__restore+0x28f/0x9ab()

On Sat, Feb 27, 2016 at 01:02:11PM +0100, Ingo Molnar wrote:
> So I'm wondering, why did this commit:
> 
>   58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs
> 

Hmm, so looking at switch_fpu_prepare():

        /*
         * If the task has used the math, pre-load the FPU on xsave processors
         * or if the past 5 consecutive context-switches used math.
         */
        fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
                      new_fpu->fpstate_active &&
                      (use_eager_fpu() || new_fpu->counter > 5);
		       ^^^^^^^^^^^^^^

and later:

        if (old_fpu->fpregs_active) {

		...

                /* Don't change CR0.TS if we just switch! */
                if (fpu.preload) {
			...
                        __fpregs_activate(new_fpu);


so I can see a possible link between 58122bf1d856 and what we're seeing.

But as I've told you offlist, I couldn't confirm that this commit was
the culprit due to my simulated reproducer. So I'm thinking the 0day
guys have a more reliable one.

> trigger the warning, while it never triggered on CPUs that were already 
> eagerfpu=on for years?

That I can't explain... yet.

FWIW, the one time splat I saw, happened on an IVB machine on 32-bit
which has always been eagerfpu=on.

> There must be something we are still missing I think.

Yeah.

-- 
Regards/Gruss,
    Boris.

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