linux-kernel - Re: question about save_xstate_sig()

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Message-ID: <20150129204534.GA30530@redhat.com>
Date:	Thu, 29 Jan 2015 21:45:34 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

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.

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