linux-kernel - Re: [PATCH -mm 1/5] list.h: add list

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Message-Id: <1205620596.6422.8.camel@lappy>
Date:	Sat, 15 Mar 2008 23:36:36 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, ananth@...ibm.com,
	jkenisto@...ibm.com, linux-kernel@...r.kernel.org,
	systemtap@...rces.redhat.com, prasanna@...ibm.com,
	shaohua.li@...el.com, davem@...emloft.net, fche@...hat.com
Subject: Re: [PATCH -mm 1/5] list.h: add list_singleton

On Fri, 2008-03-14 at 18:22 -0400, Masami Hiramatsu wrote:
> Andrew Morton wrote:
> > If your usage pattern is:
> > 
> > struct foo {
> > 	...
> > 	struct list_head bar_list;	/* A list of `struct bar's */
> > };
> > 
> > struct bar {
> > 	struct list_head list;		/* Attached to foo.bar_list */
> > 	...
> > };
> > 
> > then yes, list_singleton() makes sense.
> > 
> > But in other usage patterns it does not:
> > 
> > struct foo {
> > 	struct bar *bar_list;
> > 	...
> > };
> > 
> > struct bar {
> > 	struct list_head list;		/* All the other bars go here */
> > 	...
> > };
> > 
> > In the second case, emptiness is signified by foo.bar_list==NULL.  And in
> > this case, code which does
> > 
> > 	if (foo->bar_list && list_singleton(&foo->bar_list->list))
> > 
> > will fail if there is a single item on the list!
> > 
> > The second usage pattern is uncommon and list_empty() also returns
> > misleading answers when list_heads are used this way.
> 
> I agreed. I assume that list_singleton() is used like as list_empty().
> 
> 
> > So I guess we can proceed with your list_singleton(), but I'd just like to
> > flag this possible confusion, see what people think..

May I kindly ask to please not use the singleton name like this. It does
not implement the singleton pattern and will be a great confusion for
everybody who expects it to.


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