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Subject: Re: [PATCH] percpu: initialize best_upa variable
To:     Dennis Zhou <dennis@kernel.org>
Cc:     tj@kernel.org, cl@linux.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20210515180817.1751084-1-trix@redhat.com>
 <YKHPV4QAXmaWb6jJ@google.com>
From:   Tom Rix <trix@redhat.com>
Message-ID: <dd1dabe0-73a5-8a39-ba58-bb58a1453d90@redhat.com>
Date:   Mon, 17 May 2021 06:17:47 -0700
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On 5/16/21 7:05 PM, Dennis Zhou wrote:
> Hello,
>
> On Sat, May 15, 2021 at 11:08:17AM -0700, trix@redhat.com wrote:
>> From: Tom Rix <trix@redhat.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@redhat.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;

I was looking for initializing with something that would work.

> for (...) { }
> BUG_ON(!best_upa);
WARN_ON instead?
> 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.

Clang 10

Tom

>
> Thanks,
> Dennis
>