linux-kernel - Re: [patch 2/2] x86/fpu: Make kernel FPU protection RT friendly

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Message-ID: <20201027104207.ruurrumyg447egrk@linutronix.de>
Date:   Tue, 27 Oct 2020 11:42:07 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [patch 2/2] x86/fpu: Make kernel FPU protection RT friendly

On 2020-10-27 11:09:51 [+0100], Thomas Gleixner wrote:
> --- a/arch/x86/include/asm/fpu/api.h
> +++ b/arch/x86/include/asm/fpu/api.h
> @@ -32,15 +32,27 @@ extern void fpregs_mark_activate(void);
>   *
>   * local_bh_disable() protects against both preemption and soft interrupts
>   * on !RT kernels.
> + *
> + * On RT kernels local_bh_disable() is not sufficient because it only
> + * serializes soft interrupt related sections via a local lock, but stays
> + * preemptible. Disabling preemption is the right choice here as bottom
> + * half processing is always in thread context on RT kernels so it
> + * implicitly prevents bottom half processing as well.

The important part is that kernel_fpu_begin() also disables preemption
and it may run in softirq. It does not use fpregs_lock() and
fpregs_lock() serializes against kernel_fpu_begin().

>   */
>  static inline void fpregs_lock(void)
>  {
> -	local_bh_disable();
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		local_bh_disable();
> +	else
> +		preempt_disable();

Could you please swap that to
	if (IS_ENABLED(CONFIG_PREEMPT_RT))
		preempt_disable();
	else
		local_bh_disable();

>  }
>  

Sebastian