linux-kernel - Re: [PATCH] x86: mmiotrace: Use cpumask_available for cpumask_var

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Date:   Mon, 18 May 2020 02:31:17 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        clang-built-linux@...glegroups.com,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH] x86: mmiotrace: Use cpumask_available for cpumask_var_t
 variables

On Wed, Apr 08, 2020 at 01:53:23PM -0700, Nathan Chancellor wrote:
> When building with Clang + -Wtautological-compare and
> CONFIG_CPUMASK_OFFSTACK unset:
> 
> arch/x86/mm/mmio-mod.c:375:6: warning: comparison of array 'downed_cpus'
> equal to a null pointer is always false [-Wtautological-pointer-compare]
>         if (downed_cpus == NULL &&
>             ^~~~~~~~~~~    ~~~~
> arch/x86/mm/mmio-mod.c:405:6: warning: comparison of array 'downed_cpus'
> equal to a null pointer is always false [-Wtautological-pointer-compare]
>         if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
>             ^~~~~~~~~~~    ~~~~
> 2 warnings generated.
> 
> Commit f7e30f01a9e2 ("cpumask: Add helper cpumask_available()") added
> cpumask_available to fix warnings of this nature. Use that here so that
> clang does not warn regardless of CONFIG_CPUMASK_OFFSTACK's value.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/982
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  arch/x86/mm/mmio-mod.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/mm/mmio-mod.c b/arch/x86/mm/mmio-mod.c
> index 109325d77b3e..43fd19b3f118 100644
> --- a/arch/x86/mm/mmio-mod.c
> +++ b/arch/x86/mm/mmio-mod.c
> @@ -372,7 +372,7 @@ static void enter_uniprocessor(void)
>  	int cpu;
>  	int err;
>  
> -	if (downed_cpus == NULL &&
> +	if (!cpumask_available(downed_cpus) &&
>  	    !alloc_cpumask_var(&downed_cpus, GFP_KERNEL)) {
>  		pr_notice("Failed to allocate mask\n");
>  		goto out;
> @@ -402,7 +402,7 @@ static void leave_uniprocessor(void)
>  	int cpu;
>  	int err;
>  
> -	if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
> +	if (!cpumask_available(downed_cpus) || cpumask_weight(downed_cpus) == 0)
>  		return;
>  	pr_notice("Re-enabling CPUs...\n");
>  	for_each_cpu(cpu, downed_cpus) {
> 
> base-commit: ae46d2aa6a7fbe8ca0946f24b061b6ccdc6c3f25
> -- 
> 2.26.0
> 

Gentle ping for acceptance, I am not sure who should take this.

Cheers,
Nathan