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

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Date:	Tue, 28 Oct 2008 09:28:14 +0900
From:	Ian Kent <raven@...maw.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 Mon, 2008-10-27 at 13:40 -0700, Andrew Morton wrote:
> 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.
> 

OK, more work needed then.
Ian


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