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

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Message-ID: <20150306153622.GA26939@redhat.com>
Date:	Fri, 6 Mar 2015 16:36:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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, David Vrabel wrote:
>
> On 06/03/15 14:01, Oleg Nesterov wrote:
>
> > 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?

This needs more discussion, but in short so far I think that fpu_alloc()
from #NM exception is fine if user_mode(regs) == T.

Just do_device_not_available() should simply do conditional_sti(), I think.
Perhaps it can even enable irqs unconditionally, but we need to verify that
this is 100% correct.

And I agree that "if (!tsk_used_math(tsk))" code in math_state_restore()
should be removed. But not to avoid the allocation, and this needs other
changes.

Oleg.

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