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

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Message-ID: <alpine.DEB.2.21.1806190820210.1692@nanos.tec.linutronix.de>
Date:   Tue, 19 Jun 2018 08:23:40 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     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 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.

Thanks,

	tglx