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

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Date:	Sun, 3 May 2015 19:39:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bobby Powers <bobbypowers@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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()

On 05/02, Bobby Powers wrote:
>
> 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?

Yes, please, 4.1 needs this patch.

Borislav, perhaps you can help ?

>
> 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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