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Date: Sun, 19 Oct 2008 22:10:23 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: paulmck@...ux.vnet.ibm.com, serue@...ibm.com
Cc: sds@...ho.nsa.gov, jmorris@...ei.org, chrisw@...s-sol.org,
dhowells@...hat.com, linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
takedakn@...data.co.jp, haradats@...data.co.jp,
penguin-kernel@...ove.sakura.ne.jp
Subject: Re: [TOMOYO #10 (linux-next) 7/8] File operation restriction part.
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@...ove.SAKURA.ne.jp>
---
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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