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Date: Fri, 6 Mar 2015 19:15:25 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@amacapital.net>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly
	disable irqs
Message-ID: <20150306181525.GA1896@redhat.com>
References: <54F74F59.5070107@intel.com> <20150305195127.GA12657@redhat.com> <20150305195149.GB12657@redhat.com> <20150305201101.GA21571@gmail.com> <20150305212532.GA16890@redhat.com> <20150306075833.GA623@gmail.com> <CA+55aFzaQqR7xcHSoW_bwb_t6Cf4wiJ9LyRw3i_GSRHiGRvfDA@mail.gmail.com>
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On 03/06, Linus Torvalds wrote:
>
> On Thu, Mar 5, 2015 at 11:58 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > math_state_restore() was historically called with irqs disabled,
> > because that's how the hardware generates the trap, and also because
> > back in the days it was possible for it to be an asynchronous
> > interrupt and interrupt handlers run with irqs off.
> >
> > These days it's always an instruction trap, and furthermore it does
> > inevitably complex things such as memory allocation and signal
> > processing, which is not done with irqs disabled.
> >
> > So keep irqs enabled.
>
> I agree with the "keep irqs enabled".

Me too, but not for stable. This patch is wrong without other changes.

> IOW, I think the starting point should be something like the attached
> (which doesn't do the WARN_ON_ONCE() - it should be added for
> debugging).

Yes, agreed.

And. Even if we forget about stable, we need some minor changes before
this one. At least we need to add preempt_disable() into kernel_fpu_disable().


So I still think that the horrible hack I sent makes sense for -stable.
Just we need to cleanup (kill) it "immediately".

Oleg.