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

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Message-ID: <20150313145542.GD21603@redhat.com>
Date:	Fri, 13 Mar 2015 15:55:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Borislav Petkov <bp@...e.de>
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 03/13, Borislav Petkov wrote:
>
> 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...

Hmm. The changelog could be more clear. I'll send v2.

But please look at drop_init_fpu(). If eagerfpu == F it calls drop_fpu() and
this is what we need. flush_thread() already has the "if (!use_eager_fpu())",
we can shift drop_fpu() there.

Otherwise, if eagerfpu == T, drop_init_fpu() does restore_init_xstate() and
this just burns CPU. Until flush_thread user_has_fpu/used_math this state
restore_init_xstate() is pointless, this state will be lost after preemption.

> > +	} 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.

Agreed, will do.

Oleg.

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