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Date: Thu, 29 Jan 2015 16:53:52 -0500
From: Rik van Riel <riel@redhat.com>
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To: Oleg Nesterov <oleg@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()
References: <54C2A245.4010307@redhat.com> <20150129210723.GA31584@redhat.com> <20150129210816.GC31584@redhat.com> <54CAA7F6.2000206@redhat.com> <20150129214948.GA1045@redhat.com>
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On 01/29/2015 04:49 PM, Oleg Nesterov wrote:

> 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().

I should have known that.

Especially considering I have a patch here that
converts "tsk->thread.fpu.last_cpu = ~0" into
a disable_lazy_restore() call...