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Date: Fri, 13 Mar 2015 15:55:42 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in
	flush_thread()
Message-ID: <20150313145542.GD21603@redhat.com>
References: <54F74F59.5070107@intel.com> <20150311173346.GB5032@redhat.com> <20150311173507.GF5032@redhat.com> <20150313105251.GD31998@pd.tnic>
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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.