On Thu, 23 Oct 2008 10:35:32 +0800
Ian Kent <raven@xxxxxxxxxx> 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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