linux-kernel - Re: [PATCH] x86: AMD-perf_event: Delete unnecessary checks before the function call "free

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Date:	Mon, 16 Nov 2015 10:17:58 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	SF Markus Elfring <elfring@...rs.sourceforge.net>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org,
	Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH] x86: AMD-perf_event: Delete unnecessary checks before
 the function call "free_percpu"

On Mon, Nov 16, 2015 at 09:04:53AM +0100, SF Markus Elfring wrote:
> From: Markus Elfring <elfring@...rs.sourceforge.net>
> Date: Mon, 16 Nov 2015 09:00:20 +0100
> 
> The free_percpu() function tests whether its argument is NULL and then
> returns immediately. Thus the test around the calls is not needed.
> 
> This issue was detected by using the Coccinelle software.
> 
> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> ---
>  arch/x86/kernel/cpu/perf_event_amd_uncore.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_amd_uncore.c b/arch/x86/kernel/cpu/perf_event_amd_uncore.c
> index cc6cedb..240ecee 100644
> --- a/arch/x86/kernel/cpu/perf_event_amd_uncore.c
> +++ b/arch/x86/kernel/cpu/perf_event_amd_uncore.c
> @@ -588,11 +588,9 @@ fail_online:
>  fail_l2:
>  	if (cpu_has_perfctr_nb)
>  		perf_pmu_unregister(&amd_nb_pmu);
> -	if (amd_uncore_l2)
> -		free_percpu(amd_uncore_l2);
> +	free_percpu(amd_uncore_l2);
>  fail_nb:
> -	if (amd_uncore_nb)
> -		free_percpu(amd_uncore_nb);
> +	free_percpu(amd_uncore_nb);


So I'm really in two minds about such patches; yes its correct. But at
the same time; this isn't a performance critical piece of code and the
additional condition isn't hurting anything.

Furthermore, I find the explicit test conceptually easier than
remembering that kfree() works this way (while many other resource
freeing functions do not).

And in error paths -- which aren't our best code by far -- obvious safe
is far preferred to clever.

Ingo, preference?
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