Subject: EMM: Require single threaded execution for register and unregister

We can avoid the concurrency issues arising at registration if we
only allow registration of notifiers when the process has only a single
thread. That even allows to avoid the use of rcu.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 mm/rmap.c |   42 +++++++++++++++++++++++++++++++++---------
 1 file changed, 33 insertions(+), 9 deletions(-)

Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c	2008-04-02 13:53:46.002473685 -0700
+++ linux-2.6/mm/rmap.c	2008-04-02 14:10:20.535752527 -0700
@@ -286,20 +286,44 @@ void emm_notifier_release(struct mm_stru
 	}
 }
 
-/* Register a notifier */
+/*
+ * Register a notifier
+ *
+ * Process must be single threaded.
+ */
 void emm_notifier_register(struct emm_notifier *e, struct mm_struct *mm)
 {
+	BUG_ON(atomic_read(&mm->mm_users) != 1);
+
 	e->next = mm->emm_notifier;
-	/*
-	 * The update to emm_notifier (e->next) must be visible
-	 * before the pointer becomes visible.
-	 * rcu_assign_pointer() does exactly what we need.
-	 */
-	rcu_assign_pointer(mm->emm_notifier, e);
+	mm->emm_notifier = e;
 }
 EXPORT_SYMBOL_GPL(emm_notifier_register);
 
 /*
+ * Unregister a notifier
+ *
+ * Process must be single threaded
+ */
+void emm_notifier_unregister(struct emm_notifier *e, struct mm_struct *mm)
+{
+	struct emm_notifier *p = mm->emm_notifier;
+
+	BUG_ON(atomic_read(&mm->mm_users) != 1);
+
+	if (e == p)
+		mm->emm_notifier = e->next;
+	else {
+		while (p->next != e)
+			p = p->next;
+
+		p->next = e->next;
+	}
+	e->callback(e, mm, emm_release, 0, TASK_SIZE);
+}
+EXPORT_SYMBOL_GPL(emm_notifier_unregister);
+
+/*
  * Perform a callback
  *
  * The return of this function is either a negative error of the first
@@ -309,7 +333,7 @@ EXPORT_SYMBOL_GPL(emm_notifier_register)
 int __emm_notify(struct mm_struct *mm, enum emm_operation op,
 		unsigned long start, unsigned long end)
 {
-	struct emm_notifier *e = rcu_dereference(mm->emm_notifier);
+	struct emm_notifier *e = mm->emm_notifier;
 	int x;
 	int result = 0;
 
@@ -335,7 +359,7 @@ int __emm_notify(struct mm_struct *mm, e
 		 * emm_notifier contents (e) must be fetched after
 		 * the retrival of the pointer to the notifier.
 		 */
-		e = rcu_dereference(e->next);
+		e = e->next;
 	}
 	return result;
 }