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 15:01:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>
Cc:	Dave Hansen <dave.hansen@...el.com>, Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 03/06, Ingo Molnar wrote:
> > >
> > > * Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > > [...] The patch above looks "obviously safe", but perhaps I am
> > > > paranoid too much...
> > >
> > > IMHO your hack above isn't really acceptable, even for a backport.
> > > So lets test the patch below (assuming it's the right thing to do)
> > > and move forward?
> >
> > OK, but please note that this patch is not beckportable. If you think
> > that -stable doesn't need this fix, then I agree.
> >
> > If the caller is do_device_not_available(), then we can not enable
> > irqs before __thread_fpu_begin() + restore_fpu_checking().
> >
> > 1. Preemption in between can destroy ->fpu.state initialized by
> >    fpu_finit(), __switch_to() will save the live (wrong) FPU state
> >    again.
> >
> > 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is
> >    not nice too. It will do __save_init_fpu() and this overwrites
> >    ->fpu.state too.
> >
> > Starting from v4.0 it does kernel_fpu_disable(), but the older kernels
> > do not.
> >
> > Ingo, this code is really horrible and fragile. We need to cleanup it
> > step-by-step, imho.
>
> How about the patch from David Vrabel? That seems to solve the
> irq-disable problem too, right?

I wasn't cc'ed, I guess you mean

	[PATCHv4] x86, fpu: remove the logic of non-eager fpu mem allocation at the first usage
	http://marc.info/?l=linux-kernel&m=142564237705311&w=2

Not sure I understand it correctly after the first quick look, but

1. It conflicts with the recent changes in tip/x86/fpu

2. fpu_ini() initializes current->thread.fpu.state. This looks unneeded,
   the kernel threads no longer have FPU context and do not abuse CPU.

3. I can be easily wrong, but it looks buggy... Note that
   arch_dup_task_struct() doesn't allocate child->fpu.state if
   !tsk_used_math(parent).

Add David...

No, I do not think this patch is a good idea. Perhaps I am wrong, but I
think we need other changes. And they should start from init_fpu().

Oleg.

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