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

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Date:   Thu, 27 May 2021 14:09:43 -0700
From:   Tom Rix <trix@...hat.com>
To:     Dennis Zhou <dennis@...nel.org>
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


On 5/27/21 1:24 PM, Dennis Zhou wrote:
> Hello,
>
> On Mon, May 17, 2021 at 02:39:21PM +0000, Dennis Zhou wrote:
>> On Mon, May 17, 2021 at 06:17:47AM -0700, Tom Rix wrote:
>>> On 5/16/21 7:05 PM, Dennis Zhou wrote:
>>>> 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;
>>> I was looking for initializing with something that would work.
>>>
>> I think I prefer setting it to 0 because it forces the loop to have
>> succeeded vs being able to bypass it if the for loop logic was changed.
>>
>>>> for (...) { }
>>>> BUG_ON(!best_upa);
>>> WARN_ON instead?
>> This is initialization code. So if upa == 0, it really is a problem.
>> Having 0 units per allocation is bogus.
>>
>>>> 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
> Following up here. Are you find with me making the changes and
> attributing it to you? Otherwise I can just spin another patch real
> quick.

I am fine with you respinning, no need to attribute the change to me.

If you would like a review, include me on the cc.

Thanks!

Tom

> At this point I've already sent my PR for-5.13-fixes. So I'll queue some
> fix for-5.14.
>   
> Thanks,
> Dennis
>