linux-kernel - Re: [PATCH]btrfs: avoid comparing with NULL pointer

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Date:	Wed, 27 Jan 2010 22:19:50 +0800
From:	Liuwenyi <qingshenlwy@...il.com>
To:	"Yan, Zheng " <yanzheng@...n.com>
CC:	chris.mason@...cle.com, jbacik@...hat.com, jens.axboe@...cle.com,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	linux-kernel@...r.kernel.org, strongzgy@...il.com
Subject: Re: [PATCH]btrfs: avoid comparing with NULL pointer

于 2010年01月27日 16:00, Yan, Zheng 写道:
> 2010/1/27 Liuwenyi<qingshenlwy@...il.com>:
>    
>> In this patch, I adjust the seqence of if-conditions.
>> It will assess the page->private situation.
>> First, we make sure the page->private is not null.
>> And then, we can do some with this page->private.
>>
>> ---
>> Signed-off-by: Liuwenyi<qingshenlwy@...il.com>
>> Cc: Chris Mason<chris.mason@...cle.com>
>> Cc: Yan Zheng<zheng.yan@...cle.com>
>> Cc: Josef Bacik<jbacik@...hat.com>
>> Cc: Jens Axboe<jens.axboe@...cle.com>
>> Cc: linux-btrfs@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>>
>> ---
>> fs/btrfs/disk-io.c | 4 ++--
>> 1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index 009e3bd..a300dca 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -1407,11 +1407,11 @@ static int bio_ready_for_csum(struct bio *bio)
>>
>> bio_for_each_segment(bvec, bio, i) {
>> page = bvec->bv_page;
>> - if (page->private == EXTENT_PAGE_PRIVATE) {
>> + if (!page->private) {
>> length += bvec->bv_len;
>> continue;
>> }
>> - if (!page->private) {
>> + if (page->private == EXTENT_PAGE_PRIVATE) {
>> length += bvec->bv_len;
>> continue;
>> }
>> --
>>      
> Why do you want to do this? The code is perfect safe even
> page->private is NULL. Furthermore, your patch is malformed.
>
> Yan, Zheng
>    
Yes, I agree with you, this code is safe.
But I think we should check whether the pointer is available first.
It is unnecessary judge the "(page->private == EXTENT_PAGE_PRIVATE)"
while page->private is null.


---
Best Regards,
Liuwenyi
2010-1-27

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