linux-kernel - Re: [PATCH 12/39] dm: drop null test before destroy functions

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Message-ID: <20150914140040.GA11815@redhat.com>
Date:	Mon, 14 Sep 2015 10:00:40 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	Alasdair Kergon <agk@...hat.com>,
	kernel-janitors@...r.kernel.org, Neil Brown <neilb@...e.com>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	sergey.senozhatsky@...il.com
Subject: Re: [PATCH 12/39] dm: drop null test before destroy functions

On Mon, Sep 14 2015 at  9:46am -0400,
Mikulas Patocka <mpatocka@...hat.com> wrote:

> 
> 
> On Sun, 13 Sep 2015, Julia Lawall wrote:
> 
> > Remove unneeded NULL test.
> > 
> > The semantic patch that makes this change is as follows:
> > (http://coccinelle.lip6.fr/)
> > 
> > // <smpl>
> > @@ expression x; @@
> > -if (x != NULL)
> >   \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
> > // </smpl>
> > 
> > Signed-off-by: Julia Lawall <Julia.Lawall@...6.fr>
> > 
> > ---
> >  drivers/md/dm-bufio.c              |    3 +--
> >  drivers/md/dm-cache-target.c       |    3 +--
> >  drivers/md/dm-crypt.c              |    6 ++----
> >  drivers/md/dm-io.c                 |    3 +--
> >  drivers/md/dm-log-userspace-base.c |    3 +--
> >  drivers/md/dm-region-hash.c        |    4 +---
> >  drivers/md/dm.c                    |   13 ++++---------
> >  7 files changed, 11 insertions(+), 24 deletions(-)
> > 
> > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> > index 83cc52e..8ad39b6 100644
> > --- a/drivers/md/dm-bufio.c
> > +++ b/drivers/md/dm-bufio.c
> > @@ -1864,8 +1864,7 @@ static void __exit dm_bufio_exit(void)
> >  	for (i = 0; i < ARRAY_SIZE(dm_bufio_caches); i++) {
> >  		struct kmem_cache *kc = dm_bufio_caches[i];
> >  
> > -		if (kc)
> > -			kmem_cache_destroy(kc);
> > +		kmem_cache_destroy(kc);
> >  	}
> 
> The variable here can be NULL. I don't know how did you conclude that it 
> cannot. It seems that you didn't test the patch, if you did, you'd hit 
> NULL pointer dereference here.

kmem_cache_destroy(), mempool_destroy(), etc all check for NULL and just
return.  So there is no need for the callers to check for NULL too.

Mike
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