linux-kernel - Re: [PATCH 1/3] x86, fpu: introduce per-cpu "bool in_kernel

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Date:	Thu, 15 Jan 2015 21:22:11 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [PATCH 1/3] x86, fpu: introduce per-cpu "bool in_kernel_fpu"

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On 01/15/2015 02:19 PM, Oleg Nesterov wrote:
> interrupted_kernel_fpu_idle() tries to detect if
> kernel_fpu_begin() is safe or not. In particular it should
> obviously deny the nested kernel_fpu_begin() and this logic looks
> very confusing.
> 
> If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in 
> interrupted_kernel_fpu_idle(), and b) on the fact that _begin()
> does __thread_clear_has_fpu().
> 
> Otherwise we demand that the interrupted task has no FPU if it is
> in kernel mode, this works because __kernel_fpu_begin() does clts()
> and interrupted_kernel_fpu_idle() checks X86_CR0_TS.
> 
> Add the per-cpu "bool in_kernel_fpu" variable, and change this
> code to check/set/clear it. This allows to do more cleanups and
> fixes, see the next changes.
> 
> The patch also moves WARN_ON_ONCE() under preempt_disable() just
> to make this_cpu_read() look better, this is not really needed. And
> in fact I think we should move it into __kernel_fpu_begin().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: Rik van Riel <riel@...hat.com>


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