linux-kernel - Re: [PATCH, 3.18] sleeping function called from invalid context

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Date:	Wed, 10 Dec 2014 17:53:44 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Daniel J Blueman <daniel@...ra.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH, 3.18] sleeping function called from invalid context

On Wed, Dec 10, 2014 at 5:32 PM, Rik van Riel <riel@...hat.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
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> On 12/10/2014 07:51 PM, Andy Lutomirski wrote:
>> On Wed, Dec 10, 2014 at 4:49 PM, Rik van Riel <riel@...hat.com>
>> wrote: On 12/10/2014 07:46 PM, Daniel J Blueman wrote:
>>>>> Gah. I had some non-temporal copy changes in the wrong tree.
>>>>> I'll check with a definitely clean tree and follow up if it
>>>>> still occurs.
>>
>> The exception handlers should definitely allow sleeping, so I
>> suspect those changes may be related.
>>
>>> It would be really, really nice if we could arrange for
>>> kernel_fpu_begin to be unconditionally usable in anything except
>>> NMI context.  The crypto code would be much less scary, we could
>>> make non-temporal copies safe, etc.  Can we have ponies, too?
>
> Isn't it already?
>
> I see nothing in __kernel_fpu_begin that looks like it would
> ever need to sleep.
>

It never needs to sleep, but it does need somewhere to save the
previous state.  See irq_fpu_usable.

FWIW, I don't understand what the comments above
interrupted_kernel_fpu_idle are talking about.  The issue that I
understand is:

kernel_fpu_begin()

irq:
  kernel_fpu_begin()
  use xstate
  kernel_fpu_end()

we're screwed now :(

kernel_fpu_end()

IOW we need somewhere to put the state from the thing we interrupted.
This gets extra fun if some thread does something that takes a page
fault that uses fpu that gets interrupted, etc.  Fortunately, I think
that can't happen -- kernel_fpu_begin disables preemption.  So I think
we have a maximum of one active FPU context per thread plus some
number per cpu.  Maybe we could have a percpu array of ten or twenty
xstates to handle all possible nesting.

Also, can we just delete the non-eager code some day?

--Andy
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