linux-kernel - Re: [PATCH 3/8] x86, fpu: kill save_init_fpu(), change math_error() to use unlazy

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Message-ID: <20150216210938.GN4458@pd.tnic>
Date:	Mon, 16 Feb 2015 22:09:38 +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 3/8] x86, fpu: kill save_init_fpu(), change math_error()
 to use unlazy_fpu()

On Fri, Feb 06, 2015 at 03:02:00PM -0500, riel@...hat.com wrote:
> From: Oleg Nesterov <oleg@...hat.com>
> 
> math_error() calls save_init_fpu() after conditional_sti(), this means
> that the caller can be preempted. If !use_eager_fpu() we can hit the
> WARN_ON_ONCE(!__thread_has_fpu(tsk)) and/or save the wrong FPU state.
> 
> Change math_error() to use unlazy_fpu() and kill save_init_fpu().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  arch/x86/include/asm/fpu-internal.h | 18 ------------------
>  arch/x86/kernel/traps.c             |  2 +-
>  2 files changed, 1 insertion(+), 19 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
> index 0dbc08282291..27d00e04f911 100644
> --- a/arch/x86/include/asm/fpu-internal.h
> +++ b/arch/x86/include/asm/fpu-internal.h
> @@ -520,24 +520,6 @@ static inline void __save_fpu(struct task_struct *tsk)
>  }
>  
>  /*
> - * These disable preemption on their own and are safe
> - */
> -static inline void save_init_fpu(struct task_struct *tsk)
> -{
> -	WARN_ON_ONCE(!__thread_has_fpu(tsk));
> -
> -	if (use_eager_fpu()) {
> -		__save_fpu(tsk);
> -		return;
> -	}
> -
> -	preempt_disable();
> -	__save_init_fpu(tsk);
> -	__thread_fpu_end(tsk);
> -	preempt_enable();
> -}
> -
> -/*
>   * i387 state interaction
>   */
>  static inline unsigned short get_fpu_cwd(struct task_struct *tsk)
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index fb4cb6adf225..51c465846f06 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -663,7 +663,7 @@ static void math_error(struct pt_regs *regs, int error_code, int trapnr)
>  	/*
>  	 * Save the info for the exception handler and clear the error.
>  	 */
> -	save_init_fpu(task);
> +	unlazy_fpu(task);

Do I see it correctly that even with this there's a not-so-small hole
*after* conditional_sti() and *before* unlazy_fpu() where caller can
still get preempted?

Thanks.

-- 
Regards/Gruss,
    Boris.

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