linux-kernel - Re: [PATCH] x86: Call fixup_exception() before notify_die() in math

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Date:   Tue, 19 Jun 2018 08:23:03 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Siarhei.Liakh@...current-rt.com,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bpetkov@...e.de>
Subject: Re: [PATCH] x86: Call fixup_exception() before notify_die() in math_error()



> On Jun 18, 2018, at 11:23 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Mon, 18 Jun 2018, Andy Lutomirski wrote:
>> 
>> On Thu, Jun 14, 2018 at 10:10 PM Siarhei Liakh
>> <Siarhei.Liakh@...current-rt.com> wrote:
>>> 
>>> fpu__drop() has an explicit fwait which under some conditions can trigger
>>> a fixable FPU exception while in kernel. Thus, we should attempt to fixup
>>> the exception first, and only call notify_die() if the fixup failed just
>>> like in do_general_protection(). The original call sequence incorrectly
>>> triggers KDB entry on debug kernels under particular FPU-intensive
>>> workloads. This issue had been privately observed, fixed, and tested
>>> on 4.9.98, while this patch brings the fix to the upstream.
>> 
>> Reviewed-by: Andy Lutomirski <luto@...nel.org>
>> 
>> With the caveat that you are perpetuating what is arguably a bug in
>> some of the other entries: math_error() can now be called with IRQs
>> off and return with IRQs on.  If we actually start asserting good
>> behavior in the entry code, we'll need to fix this.
> 
> Confused. math_error() is still invoked with interrupts off. What's
> different now is that notify_die() is called with interrupts conditionally
> enabled while upstream it's always called with interrupts disabled.

True, but I don’t think that matters. What I’m grumbling about is that we can do cond_local_irq_enable() and then return without local_irq_disable().

Anyway, I think the patch is fine as is. We can unsuck the entry IRQ handling another day.

> 
> Thanks,
> 
>    tglx
> 
> 
>