Re: [PATCH] Notifier for significant events on i386

From: Stephen Hemminger (shemminger@osdl.org)
Date: Wed Dec 11 2002 - 19:19:44 EST

On Wed, 2002-12-11 at 13:15, Alan Cox wrote:
> On Wed, 2002-12-11 at 20:27, John Levon wrote:
> > There are notifiers being used that sleep inside the called notifiers.
> >
> > You could easily make a __notifier_call_chain that is lockless and
> > another one that readlocks the notifier_lock ...
>
> The notifier chains assume the users will do the locking needed for
> them. It might be possible to do cool things there with RCU

This patch changes notifier to use RCU. No interface change, just a little
more memory in each notifier_block. Also some formatting cleanup.
Please review and give comments.

diff -Nru a/include/linux/notifier.h b/include/linux/notifier.h
--- a/include/linux/notifier.h Wed Dec 11 15:46:05 2002
+++ b/include/linux/notifier.h Wed Dec 11 15:46:05 2002
@@ -9,13 +9,19 @@
  
 #ifndef _LINUX_NOTIFIER_H
 #define _LINUX_NOTIFIER_H
+
 #include <linux/errno.h>
+#include <linux/rcupdate.h>
+#include <linux/completion.h>
 
 struct notifier_block
 {
         int (*notifier_call)(struct notifier_block *self, unsigned long, void *);
         struct notifier_block *next;
         int priority;
+
+ struct rcu_head rcu;
+ struct completion complete;
 };
 
 
diff -Nru a/kernel/sys.c b/kernel/sys.c
--- a/kernel/sys.c Wed Dec 11 15:46:05 2002
+++ b/kernel/sys.c Wed Dec 11 15:46:05 2002
@@ -78,7 +78,7 @@
  */
 
 static struct notifier_block *reboot_notifier_list;
-rwlock_t notifier_lock = RW_LOCK_UNLOCKED;
+static spinlock_t notifier_lock = SPIN_LOCK_UNLOCKED;
 
 /**
  * notifier_chain_register - Add notifier to a notifier chain
@@ -89,46 +89,60 @@
  *
  * Currently always returns zero.
  */
-
 int notifier_chain_register(struct notifier_block **list, struct notifier_block *n)
 {
- write_lock(&notifier_lock);
- while(*list)
- {
+ spin_lock(&notifier_lock);
+ while (*list) {
                 if(n->priority > (*list)->priority)
                         break;
                 list= &((*list)->next);
         }
+
         n->next = *list;
- *list=n;
- write_unlock(&notifier_lock);
+ *list = n;
+ wmb();
+
+ spin_unlock(&notifier_lock);
         return 0;
 }
 
+static void notifier_unreg_complete(void *arg) {
+ struct notifier_block *n = arg;
+
+ n->next = NULL;
+ complete(&n->complete);
+}
+
 /**
  * notifier_chain_unregister - Remove notifier from a notifier chain
- * @nl: Pointer to root list pointer
+ * @list: Pointer to root list pointer
  * @n: New entry in notifier chain
  *
  * Removes a notifier from a notifier chain.
  *
  * Returns zero on success, or %-ENOENT on failure.
  */
-
-int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n)
+int notifier_chain_unregister(struct notifier_block **list, struct notifier_block *n)
 {
- write_lock(&notifier_lock);
- while((*nl)!=NULL)
- {
- if((*nl)==n)
- {
- *nl=n->next;
- write_unlock(&notifier_lock);
+ spin_lock(&notifier_lock);
+ while(*list) {
+ if (*list == n) {
+ *list = n->next;
+ wmb();
+
+ init_completion(&n->complete);
+ call_rcu(&n->rcu, notifier_unreg_complete, n);
+ spin_unlock(&notifier_lock);
+
+ wait_for_completion(&n->complete);
+
                         return 0;
                 }
- nl=&((*nl)->next);
+
+ list = &((*list)->next);
         }
- write_unlock(&notifier_lock);
+
+ spin_unlock(&notifier_lock);
         return -ENOENT;
 }
 
@@ -147,21 +161,19 @@
  * Otherwise, the return value is the return value
  * of the last notifier function called.
  */
-
 int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
 {
- int ret=NOTIFY_DONE;
+ int ret = NOTIFY_DONE;
         struct notifier_block *nb = *n;
 
- while(nb)
- {
- ret=nb->notifier_call(nb,val,v);
- if(ret&NOTIFY_STOP_MASK)
- {
+ while (nb) {
+ ret = nb->notifier_call(nb,val,v);
+ if (ret & NOTIFY_STOP_MASK)
                         return ret;
- }
- nb=nb->next;
+ nb = nb->next;
+ read_barrier_depends();
         }
+
         return ret;
 }
 

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