linux-kernel - Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for

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Message-ID: <20200929071211.GJ2628@hirez.programming.kicks-ass.net>
Date:   Tue, 29 Sep 2020 09:12:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Balbir Singh <sblbir@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars
 in enable_l1d_flush_for_task()

On Mon, Sep 28, 2020 at 02:44:57PM +0200, Lukas Bulwahn wrote:
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 6b0f4c88b07c..90515c04d90a 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -316,7 +316,7 @@ EXPORT_SYMBOL_GPL(leave_mm);
>  
>  int enable_l1d_flush_for_task(struct task_struct *tsk)
>  {
> -	int cpu, ret = 0, i;
> +	int i;
>  
>  	/*
>  	 * Do not enable L1D_FLUSH_OUT if
> @@ -329,7 +329,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
>  			!static_cpu_has(X86_FEATURE_FLUSH_L1D))
>  		return -EINVAL;
>  
> -	cpu = get_cpu();
> +	get_cpu();
>  
>  	for_each_cpu(i, &tsk->cpus_mask) {
>  		if (cpu_data(i).smt_active == true) {
> @@ -340,7 +340,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
>  
>  	set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
>  	put_cpu();
> -	return ret;
> +	return 0;
>  }

If you don't use the return value of get_cpu(), then change it over to
preempt_{dis,en}able(), but this got me looking at the function, wtf is
that garbage supposed to do in the first place

What do we need to disable preemption for?

Please explain the desired semantics against sched_setaffinity().