From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1758904AbbA2VvH (ORCPT <rfc822;w@1wt.eu>);
	Thu, 29 Jan 2015 16:51:07 -0500
Received: from mx1.redhat.com ([209.132.183.28]:44779 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1756172AbbA2VvD (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 29 Jan 2015 16:51:03 -0500
Date: Thu, 29 Jan 2015 22:49:48 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Fenghua Yu <fenghua.yu@intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end()
	if use_eager_fpu()
Message-ID: <20150129214948.GA1045@redhat.com>
References: <54C2A245.4010307@redhat.com> <20150129210723.GA31584@redhat.com> <20150129210816.GC31584@redhat.com> <54CAA7F6.2000206@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <54CAA7F6.2000206@redhat.com>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 01/29, Rik van Riel wrote:
>
> On 01/29/2015 04:08 PM, Oleg Nesterov wrote:
> > unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
> > Unconditional __thread_fpu_end() is only correct if we know that this
> > thread can't return to user-mode and use FPU.
> >
> > Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
> > and init_fpu(current) can be only called by the coredumping thread via
> > regset->get(). But it is exported to modules, and imo this should be
> > fixed anyway.
>
> What about xfpregs_set?
>
> It looks like that code copies an entire FPU state in
> from userspace, and expects the kernel to start using
> that new FPU state.
>
> This is called from the ptrace code.

Yes. But in this case tsk != current, and we ensure that __switch_to()
was finished. wait_task_inactive().

> When we switch to the traced task, the __thread_fpu_end()
> that was called from init_fpu() ensures that
> switch_fpu_begin() will actually load the new FPU state
> from memory into the registers, and we will not take
> the fpu_lazy_restore() branch.

No. in this case we rely on "tsk->thread.fpu.last_cpu = ~0" which disables
fpu_lazy_restore().

> What am I missing?

Or me ;)

Oleg.