linux-kernel - Re: [RFC PATCH 05/11] x86,fpu: ensure FPU state is reloaded from memory if task is traced

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Date:	Tue, 13 Jan 2015 17:19:38 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	riel@...hat.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: [RFC PATCH 05/11] x86,fpu: ensure FPU state is reloaded from
	memory if task is traced

On 01/11, riel@...hat.com wrote:
>
> @@ -412,8 +412,14 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc
>  	bool preload = tsk_used_math(new) && (use_eager_fpu() ||
>  					     new->thread.fpu_counter > 5);
>  	if (__thread_has_fpu(old)) {
> -		if (!__save_init_fpu(old))
> +		/*
> +		 * Make sure the FPU state is restored from memory next time,
> +		 * if the task has an FPU exception pending, or the task's in
> +		 * memory FPU state could be changed by a debugger.
> +		 */
> +		if (!__save_init_fpu(old) || task_is_stopped_or_traced(old))
>  			cpu = ~0;

Well, if debugger wants to change FPU state, it should call init_fpu()
which resets .last_cpu ?

Plus "is_stopped" is not needed, ptracer no longer can do STOPPED -> TRACED
transition.

Oleg.

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