Re: [PATCH 2/3] md/bitmap: Delete an unnecessary check before the function call "kfree"

walter harms
2015-02-03T10:19:58Z

Am 03.02.2015 10:07, schrieb NeilBrown:
> On Tue, 03 Feb 2015 09:48:01 +0100 walter harms <wharms@bfs.de> wrote:
>
>>
>>
>> Am 02.02.2015 20:46, schrieb NeilBrown:
>>> On Mon, 02 Feb 2015 16:20:42 +0100 SF Markus Elfring
>>> <elfring@users.sourceforge.net> wrote:
>>>
>>>> From: Markus Elfring <elfring@users.sourceforge.net>
>>>> Date: Mon, 2 Feb 2015 15:10:57 +0100
>>>>
>>>> The kfree() function tests whether its argument is NULL and then
>>>> returns immediately. Thus the test around the call is not needed.
>>>>
>>>> * This issue was detected by using the Coccinelle software.
>>>>
>>>> * Let us also move an assignment for the variable "pages" to the place
>>>> directly before it is really needed for a loop.
>>>>
>>>> * Let us also move another kfree() call into a block which should belong
>>>> to a previous check for the variable "bp".
>>>>
>>>> Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
>>>> ---
>>>> drivers/md/bitmap.c | 10 +++++-----
>>>> 1 file changed, 5 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
>>>> index da3604e..47d72df 100644
>>>> --- a/drivers/md/bitmap.c
>>>> +++ b/drivers/md/bitmap.c
>>>> @@ -1586,15 +1586,15 @@ static void bitmap_free(struct bitmap *bitmap)
>>>> bitmap_file_unmap(&bitmap->storage);
>>>>
>>>> bp = bitmap->counts.bp;
>>>> - pages = bitmap->counts.pages;
>>>>
>>>> /* free all allocated memory */
>>>> -
>>>> - if (bp) /* deallocate the page memory */
>>>> + if (bp) { /* deallocate the page memory */
>>>> + pages = bitmap->counts.pages;
>>>> for (k = 0; k < pages; k++)
>>>> - if (bp[k].map && !bp[k].hijacked)
>>>> + if (!bp[k].hijacked)
>>>> kfree(bp[k].map);
>>>> - kfree(bp);
>>>> + kfree(bp);
>>>> + }
>>>> kfree(bitmap);
>>>> }
>>>>
>>>
>>> Hi,
>>> I'm somewhat amused that you removed a test for one kfree, but imposed a
>>> test on another. I realised the second test was already there, but why not
>>> just:
>>>
>>> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
>>> index da3604e73e8a..ad13b2e1bf1f 100644
>>> --- a/drivers/md/bitmap.c
>>> +++ b/drivers/md/bitmap.c
>>> @@ -1592,7 +1592,7 @@ static void bitmap_free(struct bitmap *bitmap)
>>>
>>> if (bp) /* deallocate the page memory */
>>> for (k = 0; k < pages; k++)
>>> - if (bp[k].map && !bp[k].hijacked)
>>> + if (!bp[k].hijacked)
>>> kfree(bp[k].map);
>>> kfree(bp);
>>> kfree(bitmap);
>>>
>>>
>>> It makes the intention of the patch much clearer.
>>>
>>> I'd probably prefer to leave the code as it is. I don't think either patch
>>> is really an improvement in readability, and readability trumps performance
>>> in places like this.
>>>
>>
>>
>> hello Neil,
>> just for my curiosity.
>> is it possible that a bp[k] exists but bp[k].hijacked is 0 ?
>> if that case: kfree(bp) would free an object in use.
>> otherwise hijacked seems useless here as it is possible to free
>> everything.
>>
>> re,
>> wh
>
> If .hijacked is 0, the .map is a pointer (that would need to be freed) or is
> NULL.
> If .hijacked is 1, then .map has been re-tasked as 2 16-bit counters. They
> are probably bother zero at this point, but I think it is safer to completely
> skip the 'free' if 'hijacked' is set as in that case 'map' isn't a pointer.
>

It seems i was not clear enough, sorry about that.

we have a loop to kfree(bp[k].map), thats fine how it is done.

I am wondering about the kfree(bp) after the loop.
if bp[k].map is not freed for what ever reason how can it be freed later
after a kfree(bp) ?

what is what i am missing ? Is there a hidden copy ?

re,
wh