linux-kernel - Re: [PATCH] UBIFS: Remove some dead code

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Date:   Tue, 1 Nov 2016 20:17:52 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Richard Weinberger <richard@....at>, dedekind1@...il.com,
        adrian.hunter@...el.com, julia.lawall@...6.fr
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] UBIFS: Remove some dead code

Le 01/11/2016 à 12:42, Richard Weinberger a écrit :
> On 01.11.2016 07:45, Christophe JAILLET wrote:
>> 'ubifs_fast_find_freeable()' can not return an error pointer, so this test
>> can be removed.
>>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>>   fs/ubifs/gc.c | 4 ----
>>   1 file changed, 4 deletions(-)
>>
>> diff --git a/fs/ubifs/gc.c b/fs/ubifs/gc.c
>> index e845c64b6ce1..7b35e3d6cde7 100644
>> --- a/fs/ubifs/gc.c
>> +++ b/fs/ubifs/gc.c
>> @@ -846,10 +846,6 @@ int ubifs_gc_start_commit(struct ubifs_info *c)
>>   	 */
>>   	while (1) {
>>   		lp = ubifs_fast_find_freeable(c);
>> -		if (IS_ERR(lp)) {
>> -			err = PTR_ERR(lp);
>> -			goto out;
>> -		}
> Good catch, how did you find this?
> If you have a tool/script I'd like to use it too.
>
> Thanks,
> //richard
> --

Hi,
well, it is a bit tricky.

AFAIK, coccinelle is only able to match things in a given file. Finding 
issues between 2 files can be tricky.

So first, I have built a list a functions which are likely to return 
NULL, either because they explicitly return NULL or if its return value 
is tested against NULL or not. See coccinelle script n°1 below.
Then I have built a list of functions followed by a test with IS_ERR. 
See coccinelle script n°2 below.

These 2 scripts generate 2 lists of functions.
If a function is present in the 2 files, it is likely that something is 
spurious.

Either the IS_ERR is not needed (this is the case in the patch above), 
either the return value is incorrectly checked. Could also be that NULL 
is returned but an error pointer would be a better option.

I also did more or less the same for functions that return PTR_ERR and 
functions that are not followed by a test with IS_ERR.
I can post these other scripts if wanted.

Any ideas to improve or speed-up the coccinelle scripts are welcome.
Julia ?

Best regards,
CJ

Coccinelle script n°1:
=====================
@find@
identifier f;
@@

    f(...)
    {
       ...
       return NULL;
    }

@script:python@
f << find.f;
@@

print "%s" %(f)

@find2@
identifier f;
expression x;
statement S;
@@

    x = f(...);
(
    if (x) S
|
    if (!x) S
)

@script:python@
f << find2.f;
@@

print "%s" %(f)

Coccinelle script n°2:
=====================
@find@
statement S;
type t;
t *x;
identifier f;
@@

     x = f(...);
(
     if (IS_ERR(x)) S
|
     if (!IS_ERR(x)) S
)

@script:python@
f << find.f;
@@

print "%s" %(f)