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 15:00:34 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
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 06/03/15 14:01, Oleg Nesterov wrote:
> 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

This patch is from Suresh, and was originally against 3.10, so...

> 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).

...yes. It's bit-rotted a bit.

> 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().

But the general principle of avoiding the allocation in the #NM handler
and hence avoiding the need to re-enable IRQs is still a good idea, yes?

David
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