linux-kernel - Re: [PATCH 4/6] autofs4 - make autofs type usage explicit

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Date:	Mon, 27 Oct 2008 13:40:50 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ian Kent <raven@...maw.net>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	autofs@...ux.kernel.org
Subject: Re: [PATCH 4/6] autofs4 - make autofs type usage explicit

On Thu, 23 Oct 2008 10:35:32 +0800
Ian Kent <raven@...maw.net> wrote:

> This patch further improves autofs mount type usage and provides
> supplementry explanation of the changes made in the previous patch
> "autofs4 - cleanup autofs mount type usage".
> 
> Changes introduced in "autofs4 - cleanup autofs mount type usage":
> 
> - the type assigned at mount when no type is given is changed
>   from 0 to AUTOFS_TYPE_INDIRECT. This was done because 0 and
>   AUTOFS_TYPE_INDIRECT were being treated implicitly as the same
>   type.
> 
> - previously, an offset mount had it's type set to
>   AUTOFS_TYPE_DIRECT|AUTOFS_TYPE_OFFSET but the mount control
>   re-implementation needs to be able distinguish all three types.
>   So this was changed to make the type setting explicit.
> 
> - a type AUTOFS_TYPE_ANY was added for use by the re-implementation
>   when checking if a given path is a mountpoint. It's not really a
>   type as we use this to ask if a given path is a mountpoint in the
>   autofs_dev_ioctl_ismountpoint() function.
> 
> Changes introduced in this patch:
> 
> - macros to set and test the autofs mount types have been added to
>   improve readability and make the type usage explicit.

    ^^^^^^^^^^^^^^^^^^^  <<-- ??

> - the mount type is used from user space for the mount control
>   re-implementtion so, for consistency, all the definitions have
>   been moved to the user space include file include/linux/auto_fs4.h.
>
> ...
> 
> -		if (sbi->type == AUTOFS_TYPE_INDIRECT)
> +		if (autofs_type_indirect(sbi->type))

spose so.

> -			*type = AUTOFS_TYPE_INDIRECT;
> +			set_autofs_type_indirect(*type);

That's pretty nasty.  One doesn't expect a "function" to modify a
variable which was passed by value.

This interface _requires_ that set_autofs_type_indirect() be
implemented as a macro.

This didn't improve readability.

>
> ...
>
> +#define set_autofs_type_indirect(type)		(type = AUTOFS_TYPE_INDIRECT)

You'll find very few places in the kernel pull tricks like this, for
good reasons.  The obnoxious exceptions include local_irq_save() and
friends.

> +#define autofs_type_indirect(type)		(type == AUTOFS_TYPE_INDIRECT)

I guess that's OK.

But why was it implemented as a macro?  It didn't _need_ to be
implemented in cpp - it could have been implemented in C.

> +
> +#define set_autofs_type_direct(type)		(type = AUTOFS_TYPE_DIRECT)
> +#define autofs_type_direct(type)		(type == AUTOFS_TYPE_DIRECT)
> +
> +#define set_autofs_type_offset(type)		(type = AUTOFS_TYPE_OFFSET)
> +#define autofs_type_offset(type)		(type == AUTOFS_TYPE_OFFSET)
> +
> +#define autofs_type_trigger(type) \
> +	(type == AUTOFS_TYPE_DIRECT || type == AUTOFS_TYPE_OFFSET)

And this one is dangerous.  If passed an expression-with-side-effects
it will evaluate that expression either once or twice.  Bad.  Should be
implemented in C.

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