linux-kernel - Re: [PATCH v2] x86/fpu: always restore_xinit_state() when use_eager

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Date:	Mon, 27 Apr 2015 17:19:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bobby Powers <bobbypowers@...il.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>, Borislav Petkov <bp@...e.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>
Subject: Re: [PATCH v2] x86/fpu: always restore_xinit_state() when
	use_eager_cpu()

Thanks!

On 04/27, Bobby Powers wrote:
>
> v2: switch used_math() -> tsk_used_math(tsk) to consistently use the
> grabbed tsk instead of current, like in the rest of flush_thread().
>
> Oleg's commit f893959b ("x86/fpu: Don't abuse drop_init_fpu() in
> flush_thread()") removed drop_init_fpu() usage from flush_thread.
> This seems to break things for me - the Go 1.4 test suite fails all
> over the place with floating point comparision errors (offending
> commit found through bisection).
>
> The functional change was that flush_thread after f893959b only calls
> restore_init_xstate when both use_eager_fpu() and !used_math() are
> true.  drop_init_fpu (now fpu_reset_state) calls restore_init_xstate()
> regardless of whether current used_math() - apply the same logic here.
>
> Fixes: f893959b ("x86/fpu: Don't abuse drop_init_fpu() in flush_thread()")
> Signed-off-by: Bobby Powers <bobbypowers@...il.com>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Pekka Riikonen <priikone@....fi>
> Cc: Quentin Casasnovas <quentin.casasnovas@...cle.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Suresh Siddha <sbsiddha@...il.com>
> ---
>  arch/x86/kernel/process.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 8213da6..1a6fcf8 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -156,11 +156,13 @@ void flush_thread(void)
>  		/* FPU state will be reallocated lazily at the first use. */
>  		drop_fpu(tsk);
>  		free_thread_xstate(tsk);
> -	} else if (!used_math()) {
> -		/* kthread execs. TODO: cleanup this horror. */
> -		if (WARN_ON(init_fpu(tsk)))
> -			force_sig(SIGKILL, tsk);
> -		user_fpu_begin();
> +	} else {
> +		if (!tsk_used_math(tsk)) {
> +			/* kthread execs. TODO: cleanup this horror. */
> +			if (WARN_ON(init_fpu(tsk)))
> +				force_sig(SIGKILL, tsk);
> +			user_fpu_begin();
> +		}
>  		restore_init_xstate();
>  	}
>  }
> --
> 2.3.6
>

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