linux-kernel - Re: [PATCH 2/8] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager

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Date:	Mon, 16 Feb 2015 21:25:40 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	riel@...hat.com, oleg@...hat.com
Cc:	dave.hansen@...ux.intel.com, sbsiddha@...il.com,
	luto@...capital.net, tglx@...utronix.de, mingo@...nel.org,
	hpa@...or.com, fenghua.yu@...el.com, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if
 use_eager_fpu()

On Fri, Feb 06, 2015 at 03:01:59PM -0500, riel@...hat.com wrote:
> From: Oleg Nesterov <oleg@...hat.com>
> 
> unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
> Unconditional __thread_fpu_end() is only correct if we know that this
> thread can't return to user-mode and use FPU.
> 
> Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
> and init_fpu(current) can be only called by the coredumping thread via
> regset->get(). But it is exported to modules, and imo this should be
> fixed anyway.
> 
> And if we check use_eager_fpu() we can use __save_fpu() like fpu_copy()
> and save_init_fpu() do.
> 
> - It seems that even !use_eager_fpu() case doesn't need the unconditional
>   __thread_fpu_end(), we only need it if __save_init_fpu() returns 0.

I can follow so far.

> - It is still not clear to me if __save_init_fpu() can safely nest with
>   another save + restore from __kernel_fpu_begin(). If not, we can use
>   kernel_fpu_disable() to fix the race.

Well, my primitive understanding would say no, not safely, for the
simple reason that we have only one XSAVE state area per thread.
However, __kernel_fpu_begin() is called with preemption disabled so ...
I guess I'm still not seeing the race.

Btw, what is kernel_fpu_disable()? Can't find it here.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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