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

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Message-ID: <CA+55aFzaQqR7xcHSoW_bwb_t6Cf4wiJ9LyRw3i_GSRHiGRvfDA@mail.gmail.com>
Date:	Fri, 6 Mar 2015 09:33:46 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	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 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".

However, I do *not* agree with the actual patch, which doesn't do that at all.
> @@ -844,8 +844,9 @@ void math_state_restore(void)
>  {
>         struct task_struct *tsk = current;
>
> +       local_irq_enable();
> +

There's a big difference between "keep interrupts enabled" (ok) and
"explicitly enable interrupts in random contexts" (*NOT* ok).

So get rid of the "local_irq_enable()" entirely, and replace it with a

   WARN_ON_ONCE(irqs_disabled());

and let's just fix the cases where this actually gets called with
interrupts off. In particular, let's just make the
device_not_available thing use a trap gate, not an interrupt gate. And
then remove the "conditional_sti()" stuff.

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).

*NOT* some kind of "let's just enable interrupts blindly" approach.

This is completely untested, of course. But I don't see why we should
use an interrupt gate for this. Is there anything in
"exception_enter()" that requires it?

                       Linus

View attachment "patch.diff" of type "text/plain" (1852 bytes)