Re: [TOMOYO #10 (linux-next) 7/8] File operation restriction part.

[Tetsuo Handa]

Hello.

Paul E. McKenney wrote:
> > Maybe I'm misunderstanding what "mb()" can do.
> 
> The problem is that while wmb() and mb() do in fact order writes, they 
> cannot order the other task's reads.
> 
I expected that "mb()" can order the other task's reads.

Now, I understood that there is no room for optimizing the reader process
by omitting smp_read_barrier_depends() on read side.

OK, let's return to http://lkml.org/lkml/2008/10/14/406 .
Below is the updated version of list1 operations.
As I now use rcu_assign_pointer() and rcu_dereference() which depend on
include/linux/rcupdate.h , I separated the code from include/linux/list.h .
Did I update correctly?

---
Subject: Singly linked list implementation.

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/list1.h |   81 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

--- /dev/null
+++ linux-next/include/linux/list1.h
@@ -0,0 +1,81 @@
+#ifndef _LINUX_LIST1_H
+#define _LINUX_LIST1_H
+
+#include <linux/list.h>
+#include <linux/rcupdate.h>
+
+/*
+ * Singly linked list implementation.
+ *
+ * This list supports only two operations.
+ * (1) Append an entry to the tail of the list.
+ * (2) Read all entries starting from the head of the list.
+ *
+ * This list is designed for holding "write once, read many" entries.
+ * This list requires no locks for read operation.
+ * This list doesn't support "remove an entry from the list" operation.
+ */
+
+/* To reduce memory usage, this list doesn't use "->prev" pointer. */
+struct list1_head {
+	struct list1_head *next;
+};
+
+#define LIST1_HEAD_INIT(name) { &(name) }
+#define LIST1_HEAD(name) struct list1_head name = LIST1_HEAD_INIT(name)
+
+static inline void INIT_LIST1_HEAD(struct list1_head *list)
+{
+	list->next = list;
+}
+
+/* Reuse list_entry because it doesn't use "->prev" pointer. */
+#define list1_entry list_entry
+
+/* Reuse list_for_each_rcu because it doesn't use "->prev" pointer. */
+#define list1_for_each list_for_each_rcu
+/* Reuse list_for_each_entry_rcu because it doesn't use "->prev" pointer. */
+#define list1_for_each_entry list_for_each_entry_rcu
+
+/**
+ * list1_for_each_cookie - iterate over a list with cookie.
+ * @pos:        the &struct list1_head to use as a loop cursor.
+ * @cookie:     the &struct list1_head to use as a cookie.
+ * @head:       the head for your list.
+ *
+ * Same with list_for_each_rcu() except that this primitive uses @cookie
+ * so that we can continue iteration.
+ * @cookie must be NULL when iteration starts, and @cookie will become
+ * NULL when iteration finishes.
+ *
+ * Since list elements are never removed, we don't need to get a lock
+ * or a reference count.
+ */
+#define list1_for_each_cookie(pos, cookie, head)                      \
+	for (({ if (!cookie)                                          \
+				     cookie = head; }),               \
+	     pos = rcu_dereference((cookie)->next);                   \
+	     prefetch(pos->next), pos != (head) || ((cookie) = NULL); \
+	     (cookie) = pos, pos = rcu_dereference(pos->next))
+
+/**
+ * list1_add_tail - add a new entry to list1 list.
+ * @new: new entry to be added.
+ * @head: list head to add it before.
+ *
+ * Same with list_add_tail_rcu() without "->prev" pointer.
+ *
+ * Caller must hold a lock for protecting @head.
+ */
+static inline void list1_add_tail(struct list1_head *new,
+				  struct list1_head *head)
+{
+	struct list1_head *prev = head;
+
+	new->next = head;
+	while (prev->next != head)
+		prev = prev->next;
+	rcu_assign_pointer(prev->next, new);
+}
+
+#endif
---

By the way, quoting from ordering.2007.09.19a.pdf :

| One could place an smp_rmb() primitive between the pointer fetch and
| dereference. However, this imposes unneeded overhead on systems (such as
| i386, IA64, PPC, and SPARC) that respect data dependencies on the read side.
| A smp_read_barrier_depends() primitive has been added to the Linux 2.6 kernel
| to eliminate overhead on these systems.

In 2.4 kernels, to support Alpha architecture, people use smp_rmb() which
imposes unneeded overhead on non Alpha architecture.
In 2.6 kernels, to support Alpha architecture, people use
smp_read_barrier_depends() which does not impose unneeded overhead on
non Alpha architecture.
That's nice.

| Alpha is the only CPU where smp_read_barrier_depends() is an smp_mb() rather
| than a no-op.

I found

  #define smp_read_barrier_depends()      read_barrier_depends()

in arch/h8300/include/asm/system.h but couldn't find the definition of
read_barrier_depends() within that file.
I hope read_barrier_depends() is defined as a no-op by some other header files.

Regards.
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