linux-kernel - Re: [PATCH 2/3] x86/fpu: Simplify kernel_fpu

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Date:   Tue, 4 Jun 2019 13:47:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     x86@...nel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/fpu: Simplify kernel_fpu_begin

On Tue, Jun 04, 2019 at 09:15:23AM +0200, Christoph Hellwig wrote:
> +void kernel_fpu_begin(void)
>  {
> +	preempt_disable();
>  
>  	WARN_ON_FPU(!irq_fpu_usable());
> +	WARN_ON_FPU(this_cpu_read(in_kernel_fpu));
>  
> +	this_cpu_write(in_kernel_fpu, true);
>  
> +	if (current->mm && !test_thread_flag(TIF_NEED_FPU_LOAD)) {

Did that want to be: !(current->flags & PF_KTHREAD), instead?

Because I'm thinking that kernel_fpu_begin() on a kthread that has
use_mm() employed shouldn't be doing this..

> +		set_thread_flag(TIF_NEED_FPU_LOAD);
> +		/*
> +		 * Ignore return value -- we don't care if reg state
> +		 * is clobbered.
> +		 */
> +		copy_fpregs_to_fpstate(&current->thread.fpu);
>  	}
>  	__cpu_invalidate_fpregs_state();
>  }