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

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Message-ID: <YKKAGbOyRSX5jmxY@google.com>
Date:   Mon, 17 May 2021 14:39:21 +0000
From:   Dennis Zhou <dennis@...nel.org>
To:     Tom Rix <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

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