linux-kernel - Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush

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Date:	Fri, 13 Mar 2015 11:52:51 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	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 4/4] x86/fpu: don't abuse drop_init_fpu() in
 flush_thread()

On Wed, Mar 11, 2015 at 06:35:07PM +0100, Oleg Nesterov wrote:
> drop_init_fpu() makes no sense. We need drop_fpu() and only if

Oh, please explain why. I can try to rhyme it up as something like "we
don't need to restore FPU context when flushing the thread" but I'm not
sure...

> !use_eager_fpu().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  arch/x86/kernel/process.c |   11 ++++-------
>  1 files changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index c396de2..2e71120 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -131,14 +131,11 @@ void flush_thread(void)
>  	flush_ptrace_hw_breakpoint(tsk);
>  	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
>  
> -	drop_init_fpu(tsk);
> -	/*
> -	 * Free the FPU state for non xsave platforms. They get reallocated
> -	 * lazily at the first use.
> -	 */
> -	if (!use_eager_fpu())
> +	if (!use_eager_fpu()) {
> +		/* FPU state will be reallocated lazily at the first use. */
> +		drop_fpu(tsk);
>  		free_thread_xstate(tsk);
> -	else if (!used_math()) {
> +	} else if (!used_math()) {
>  		/* kthread execs. TODO: cleanup this horror. */
>  		if (WARN_ON(init_fpu(current)))
>  			force_sig(SIGKILL, current);

Also, can we clean up the tsk/current usage here?

We assign current to tsk and we work with it but then later use current
again. Needlessly confusing.

Thanks.

-- 
Regards/Gruss,
    Boris.

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