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Re: [PATCH 3/3] Add timeout feature

To: "Takashi Sato" <t-sato@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/3] Add timeout feature
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 27 Jun 2008 11:57:27 -0700
Cc: viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
In-reply-to: <7B349EFCD35842D4ADAEB402D2BDCA4E@xxxxxxxxxxxxxxxx>
References: <20080624160056t-sato@xxxxxxxxxxxxxxx> <20080624150925.765155f0.akpm@xxxxxxxxxxxxxxxxxxxx> <7B349EFCD35842D4ADAEB402D2BDCA4E@xxxxxxxxxxxxxxxx>
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On Fri, 27 Jun 2008 20:33:58 +0900
"Takashi Sato" <t-sato@xxxxxxxxxxxxx> wrote:

> >> - 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.

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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