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Date:	Mon, 30 Jun 2008 08:13:07 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	<viro@...IV.linux.org.uk>, <linux-ext4@...r.kernel.org>,
	<xfs@....sgi.com>, <dm-devel@...hat.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<axboe@...nel.dk>, <mtk.manpages@...glemail.com>
Subject: Re: [PATCH 3/3] Add timeout feature

Hi,

>> >>  case XFS_FSOP_GOING_FLAGS_DEFAULT: {
>> >> - struct super_block *sb = freeze_bdev(mp->m_super->s_bdev);
>> >> + struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, 0);
>> >
>> > Using NULL here is clearer and will, I expect, avoid a sparse warning.
>> 
>> I checked it but I couldn't find a sparse warning in xfs_fsops.c.
>> Can you tell me how to use NULL?
> 
> struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, NULL);
> 
> :)
> 
> It's much better to use NULL here rather than literal zero because the
> reader of this code can then say "ah-hah, we're passing in a pointer". 
> Whereas plain old "0" could be a pointer or a scalar.

The second argument's type of freeze_bdev() is "long", not pointer as below.
struct super_block *freeze_bdev(struct block_device *, long timeout_msec);

So "0" is reasonable, isn't it?

> We should always use NULL to represent a null pointer in the kernel. 
> The one acceptable exception is when testing for nullness:
> 
> if (ptr1)
> if (!ptr2)
> 
> Often people will use
> 
> if (ptr1 != NULL)
> if (ptr2 == NULL)
> 
> in this case as well.  (I prefer the shorter version personally, but
> either is OK).

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