linux-kernel - Re: [PATCH] percpu: initialize best

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Date:   Mon, 17 May 2021 02:05:11 +0000
From:   Dennis Zhou <dennis@...nel.org>
To:     trix@...hat.com
Cc:     tj@...nel.org, cl@...ux.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] percpu: initialize best_upa variable

Hello,

On Sat, May 15, 2021 at 11:08:17AM -0700, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> Static analysis reports this problem
> percpu.c:2945:6: warning: Assigned value is garbage or undefined
>         upa = best_upa;
>             ^ ~~~~~~~~
> best_upa may not be set, so initialize it.
> 
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  mm/percpu.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index a257c3efdf18b..6578b706fae81 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -2916,6 +2916,7 @@ static struct pcpu_alloc_info * __init __flatten pcpu_build_alloc_info(
>  	 * Related to atom_size, which could be much larger than the unit_size.
>  	 */
>  	last_allocs = INT_MAX;
> +	best_upa = max_upa;
>  	for (upa = max_upa; upa; upa--) {
>  		int allocs = 0, wasted = 0;
>  
> -- 
> 2.26.3
> 

I think the proper fix would be:

best_upa = 0;
for (...) { }
BUG_ON(!best_upa);
upa = best_upa;

If you're fine with this I'll make the changes and apply it to
for-5.13-fixes.

Can you also tell me what static analysis tool produced this? I'm just a
little curious because this code hasn't changed in several years so I'd
have expected some static analyzer to have caught this by now.

Thanks,
Dennis