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Date: Thu, 29 Jan 2015 21:45:34 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
        Suresh Siddha <suresh.b.siddha@intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?
Message-ID: <20150129204534.GA30530@redhat.com>
References: <54C2A245.4010307@redhat.com> <20150124202021.GA1285@redhat.com> <54C6CD64.10208@redhat.com> <20150127194030.GA29879@redhat.com> <54C7F4BB.5020509@redhat.com>
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On 01/27, Rik van Riel wrote:
>
> On 01/27/2015 02:40 PM, Oleg Nesterov wrote:
> >>>
> >>> - Is unlazy_fpu()->__save_init_fpu() safe wrt
> >>> __kernel_fpu_begin() from irq?
>
> It looks like it should be safe, as long as __save_init_fpu()
> knows that the task no longer has the FPU after __kernel_fpu_end(),
> so it does not try to save the kernel FPU state to the user's
> task->thread.fpu.state->xstate

Not sure this is enough, but...

> The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end()
> needs to be kept from running during unlazy_fpu().

Yes,

> This means interrupted_kernel_fpu_idle and/or irq_fpu_usable
> need to check whether preemption is disabled, and lock out
> __kernel_fpu_begin() when preemption is disabled.

But we already have kernel_fpu_disable/enable. unlazy_cpu() can use
it to avoid the race ?

> I can certainly merge unlazy_fpu() and save_init_fpu() into the
> same function, but I am not sure whether or not it should call
> __thread_fpu_end() - it looks like that would be desirable in some
> cases, but not in others...

I _think_ that we never actually want __thread_fpu_end(), although it
doesn't really hurt if !eager. Probably ulazy/save should do

	if (!__save_init_fpu())
		__thread_fpu_end();

But again, this is minor.

Oleg.