[PATCH] (4/5) improved notifier callback mechanism - read copy update

From: Stephen Hemminger (shemminger@osdl.org)
Date: Wed Dec 18 2002 - 17:14:12 EST

The notifier interface was only partially locked. The
notifier_call_chain needs to be called in places where it is impossible
to safely without having deadlocks; for example, NMI watchdog timeout.

This patch uses read-copy-update to manage the list. One extra bit of
safety is using a reference count on the notifier_blocks to allow for
cases like oprofile which need to sleep in a callback.

# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or
higher.
# This patch includes the following deltas:
# ChangeSet 1.983 -> 1.984
# include/linux/notifier.h 1.7 -> 1.8
# kernel/sys.c 1.39 -> 1.40
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 02/12/18 shemminger@osdl.org 1.984
# 04-notifier-rcu.patch
# --------------------------------------------
#
diff -Nru a/include/linux/notifier.h b/include/linux/notifier.h
--- a/include/linux/notifier.h Wed Dec 18 10:00:43 2002
+++ b/include/linux/notifier.h Wed Dec 18 10:00:43 2002
@@ -11,14 +11,15 @@
 #define _LINUX_NOTIFIER_H
 
 #include <linux/list.h>
+#include <asm/atomic.h>
 
 struct notifier_block {
         int (*notifier_call)(struct notifier_block *, unsigned long, void *);
- int priority;
+ int priority;
 
+ atomic_t inuse;
         struct list_head link;
-};
-
+} ____cacheline_aligned;
 
 #ifdef __KERNEL__
 
diff -Nru a/kernel/sys.c b/kernel/sys.c
--- a/kernel/sys.c Wed Dec 18 10:00:43 2002
+++ b/kernel/sys.c Wed Dec 18 10:00:43 2002
@@ -22,6 +22,7 @@
 #include <linux/security.h>
 #include <linux/dcookies.h>
 #include <linux/suspend.h>
+#include <linux/rcupdate.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>
@@ -78,7 +79,7 @@
  */
 
 static LIST_HEAD(reboot_notifier_list);
-static rwlock_t notifier_lock = RW_LOCK_UNLOCKED;
+static spinlock_t notifier_lock = SPIN_LOCK_UNLOCKED;
 
 /**
  * notifier_chain_register - Add notifier to a notifier chain
@@ -95,7 +96,9 @@
         struct list_head *p;
 
         INIT_LIST_HEAD(&n->link);
- write_lock(&notifier_lock);
+ atomic_set(&n->inuse, 0);
+
+ spin_lock(&notifier_lock);
         list_for_each(p, list) {
                 struct notifier_block *e
                         = list_entry(p, struct notifier_block, link);
@@ -104,7 +107,7 @@
         }
 
         list_add(&n->link, p);
- write_unlock(&notifier_lock);
+ spin_unlock(&notifier_lock);
         return 0;
 }
 
@@ -122,15 +125,24 @@
 {
         struct list_head *cur;
 
- write_lock(&notifier_lock);
+ might_sleep();
+
+ spin_lock(&notifier_lock);
         list_for_each(cur, list) {
                 if (n == list_entry(cur, struct notifier_block, link)) {
- list_del(cur);
- write_unlock(&notifier_lock);
+ list_del_rcu(cur);
+ spin_unlock(&notifier_lock);
+
+ synchronize_kernel();
+
+ /* notifier may be sleeping */
+ while (atomic_read(&n->inuse) > 0)
+ yield();
+
                         return 0;
                 }
         }
- write_unlock(&notifier_lock);
+ spin_unlock(&notifier_lock);
         return -ENOENT;
 }
 
@@ -148,23 +160,31 @@
  * the notifier function which halted execution.
  * Otherwise, the return value is the return value
  * of the last notifier function called.
+ *
+ * This might be called from NMI or other context where it
+ * is impossible to sleep or spin.
  */
  
 int notifier_call_chain(struct list_head *list, unsigned long val, void
*v)
 {
- struct list_head *p;
+ struct list_head *p, *nxtp;
         int ret = NOTIFY_DONE;
 
- list_for_each(p, list) {
+ rcu_read_lock();
+ list_for_each_safe_rcu(p, nxtp, list) {
                 struct notifier_block *nb =
                         list_entry(p, struct notifier_block, link);
 
+ atomic_inc(&nb->inuse);
                 ret = nb->notifier_call(nb,val,v);
+ atomic_dec(&nb->inuse);
+
                 if (ret & NOTIFY_STOP_MASK)
                         goto end_loop;
         }
 
  end_loop:
+ rcu_read_unlock();
         return ret;
 }
 

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This archive was generated by hypermail 2b29 : Mon Dec 23 2002 - 22:00:21 EST