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

[Masami Hiramatsu <mhiramat@redhat.com>]

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..

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com