linux-kernel - Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs

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Date:	Fri, 6 Mar 2015 19:15:25 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly
	disable irqs

On 03/06, Linus Torvalds wrote:
>
> On Thu, Mar 5, 2015 at 11:58 PM, Ingo Molnar <mingo@...nel.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.

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