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

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Date:	Sat, 2 May 2015 16:42:59 -0400
From:	Bobby Powers <bobbypowers@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	x86@...nel.org, Andi Kleen <ak@...ux.intel.com>,
	Borislav Petkov <bp@...e.de>,
	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()

Hello,

Oleg Nesterov <oleg@...hat.com> wrote:
> Thanks!

Dave gave this his:

Tested-By: Dave Hansen <dave.hansen@...el.com>

over under v1 of this patch:
https://lkml.kernel.org/g/5543C277.9070208@intel.com

Anything else anyone sees, or anyone in particular I have to poke at
to get this in?  Ingo?

yours,
Bobby

On Mon, Apr 27, 2015 at 11:19 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 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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