Add list_singleton to check a list has just one entry.

list_singleton is useful to check whether a list_head which
have been temporarily allocated for listing objects can be
released or not.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
---
 include/linux/list.h |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: 2.6.25-rc5-mm1/include/linux/list.h
===================================================================
--- 2.6.25-rc5-mm1.orig/include/linux/list.h
+++ 2.6.25-rc5-mm1/include/linux/list.h
@@ -211,6 +211,15 @@ static inline int list_empty_careful(con
 	return (next == head) && (next == head->prev);
 }

+/**
+ * list_singleton - tests whether a list has just one entry.
+ * @head: the list to test.
+ */
+static inline int list_singleton(const struct list_head *head)
+{
+	return !list_empty(head) && (head->next == head->prev);
+}
+
 static inline void __list_splice(struct list_head *list,
 				 struct list_head *head)
 {
-- 
Masami Hiramatsu

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

e-mail: mhiramat@redhat.com





--

On Fri, 14 Mar 2008 16:40:36 -0400

This hurts my brain.

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.

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

--

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.


--

Hi Peter,


Indeed.

I have some other candidates.
- list_single
- list_has_one
- list_one

Could you tell me which or some other name you recommend?

Thanks,

-- 
Masami Hiramatsu

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

e-mail: mhiramat@redhat.com

--

I think list_has_one() or list_is_singular() are good names, they convey
they are a test for a condition by using a form of be.

--

Rename list_singleton to list_is_singular.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
---

OK, I picked up list_is_singular().

Thanks,

 include/linux/list.h |    4 ++--
 kernel/kprobes.c     |    4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

Index: 2.6.25-rc5-mm1/include/linux/list.h
===================================================================
--- 2.6.25-rc5-mm1.orig/include/linux/list.h
+++ 2.6.25-rc5-mm1/include/linux/list.h
@@ -212,10 +212,10 @@ static inline int list_empty_careful(con
 }

 /**
- * list_singleton - tests whether a list has just one entry.
+ * list_is_singular - tests whether a list has just one entry.
  * @head: the list to test.
  */
-static inline int list_singleton(const struct list_head *head)
+static inline int list_is_singular(const struct list_head *head)
 {
 	return !list_empty(head) && (head->next == head->prev);
 }
Index: 2.6.25-rc5-mm1/kernel/kprobes.c
===================================================================
--- 2.6.25-rc5-mm1.orig/kernel/kprobes.c
+++ 2.6.25-rc5-mm1/kernel/kprobes.c
@@ -643,7 +643,7 @@ static int __kprobes __unregister_kprobe
 valid_p:
 	if (old_p == p ||
 	    (old_p->pre_handler == aggr_pre_handler &&
-	     list_singleton(&old_p->list))) {
+	     list_is_singular(&old_p->list))) {
 		/*
 		 * Only probe on the hash list. Disarm only if kprobes are
 		 * enabled - otherwise, the breakpoint would already have
@@ -679,7 +679,7 @@ static void __kprobes __unregister_kprob
 			module_put(mod);
 	}

-	if (list_empty(&p->list) || list_singleton(&p->list)) {
+	if (list_empty(&p->list) || list_is_singular(&p->list)) {
 		if (!list_empty(&p->list)) {
 			/* "p" is the last child of an aggr_kprobe */
 			old_p = list_entry(p->list.next, struct kprobe, list);


-- 
Masami Hiramatsu

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

e-mail: mhiramat@redh...
[ message continues ]