On Wed, 7 Dec 2005, Andrew Morton wrote:
> The default version of notifier chains should be the lockless version -
> just like list_head, radix_tree, etc. (idr tries to provide its own
> locking and has turned out to be a classic case of why we shouldn't do
> that).
>
> And sure, it makes sense to provide additional, higher-level data
> structures and APIs which layer on top of that, purely as a code
> consolidation exercise. But they shouldn't be called notifier_chains.
> Call them notifier_chain_mutex_locked and notifier_chain_rwlocked and
> notifier_chain_spinlocked or whatever.
The code below defines three new data structures: atomic_notifier_head,
blocking_notifier_head, and raw_notifier_head. The first two correspond
to what we had in the earlier patch, and raw_notifier_head is almost the
same as the current implementation, with no locking or protection at all.
(None is the default; the type has to be specified when a notifier chain
is defined or used.)
Each of the three has its own API, which is layered on top of a single set
of core routines. Is this basically what you had in mind?
> As for the NMIs and RCU: I suspect it was simply a mistake to try to use
> notifier chains for NMI registration in the first place - they are simply
> too complex a data structure for this. I think I previously suggested
> removing that code and using just a fixed-size array of function pointers.
> But if the RCUified notifier chain solution is less-than-totally-gross then
> that might be OK as well.
Let me know if the atomic_notifier_head code looks totally gross. If you
think this scheme is okay, I'll go through and make the corresponding
changes to all the users of the notifier_chain API.
Alan Stern
Index: usb-2.6/include/linux/notifier.h
===================================================================
--- usb-2.6.orig/include/linux/notifier.h
+++ usb-2.6/include/linux/notifier.h
@@ -10,25 +10,100 @@
#ifndef _LINUX_NOTIFIER_H
#define _LINUX_NOTIFIER_H
#include <linux/errno.h>
+#include <asm/semaphore.h>
+#include <linux/rwsem.h>
-struct notifier_block
-{
- int (*notifier_call)(struct notifier_block *self, unsigned long, void *);
+/*
+ * Notifier chains are of three types:
+ *
+ * Atomic notifier chains: Chain callbacks run in interrupt/atomic
+ * context. Callouts are not allowed to block.
+ * Blocking notifier chains: Chain callbacks run in process context.
+ * Callouts are allowed to block.
+ * Raw notifier chains: There are no restrictions on callbacks,
+ * registration, or unregistration. All locking and protection
+ * must be provided by the caller.
+ *
+ * atomic_notifier_chain_register() and blocking_notifier_chain_register()
+ * may be called only from process context, and likewise for the
+ * corresponding _unregister() routines.
+ *
+ * atomic_notifier_chain_unregister() and blocking_notifier_chain_unregister()
+ * _must not_ be called from within the call chain.
+ */
+
+struct notifier_block {
+ int (*notifier_call)(struct notifier_block *, unsigned long, void *);
struct notifier_block *next;
int priority;
};
+struct atomic_notifier_head {
+ struct semaphore sem;
+ struct notifier_block *head;
+};
+
+struct blocking_notifier_head {
+ struct rw_semaphore rwsem;
+ struct notifier_block *head;
+};
+
+struct raw_notifier_head {
+ struct notifier_block *head;
+};
+
+#define ATOMIC_INIT_NOTIFIER_HEAD(name) do { \
+ init_MUTEX(&(name)->rwsem); \
+ (name)->head = NULL; \
+ } while (0)
+#define BLOCKING_INIT_NOTIFIER_HEAD(name) do { \
+ init_rwsem(&(name)->rwsem); \
+ (name)->head = NULL; \
+ } while (0)
+#define RAW_INIT_NOTIFIER_HEAD(name) do { \
+ (name)->head = NULL; \
+ } while (0)
+
+#define ATOMIC_NOTIFIER_HEAD(name) \
+ struct atomic_notifier_head name = { \
+ .sem = __SEMAPHORE_INITIALIZER((name).sem, 1), \
+ .head = NULL }
+#define BLOCKING_NOTIFIER_HEAD(name) \
+ struct blocking_notifier_head name = { \
+ .rwsem = __RWSEM_INITIALIZER((name).rwsem), \
+ .head = NULL }
+#define RAW_NOTIFIER_HEAD(name) \
+ struct raw_notifier_head name = { \
+ .head = NULL }
#ifdef __KERNEL__
-extern int notifier_chain_register(struct notifier_block **list, struct notifier_block *n);
-extern int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n);
-extern int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v);
+extern int atomic_notifier_chain_register(struct atomic_notifier_head *,
+ struct notifier_block *);
+extern int blocking_notifier_chain_register(struct blocking_notifier_head *,
+ struct notifier_block *);
+extern int raw_notifier_chain_register(struct raw_notifier_head *,
+ struct notifier_block *);
+
+extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *,
+ struct notifier_block *);
+extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *,
+ struct notifier_block *);
+extern int raw_notifier_chain_unregister(struct raw_notifier_head *,
+ struct notifier_block *);
+
+extern int atomic_notifier_call_chain(struct atomic_notifier_head *,
+ unsigned long val, void *v);
+extern int blocking_notifier_call_chain(struct blocking_notifier_head *,
+ unsigned long val, void *v);
+extern int raw_notifier_call_chain(struct raw_notifier_head *,
+ unsigned long val, void *v);
#define NOTIFY_DONE 0x0000 /* Don't care */
#define NOTIFY_OK 0x0001 /* Suits me */
#define NOTIFY_STOP_MASK 0x8000 /* Don't call further */
-#define NOTIFY_BAD (NOTIFY_STOP_MASK|0x0002) /* Bad/Veto action */
+#define NOTIFY_BAD (NOTIFY_STOP_MASK|0x0002)
+ /* Bad/Veto action */
/*
* Clean way to return from the notifier and stop further calls.
*/
Index: usb-2.6/kernel/sys.c
===================================================================
--- usb-2.6.orig/kernel/sys.c
+++ usb-2.6/kernel/sys.c
@@ -94,67 +94,192 @@ int cad_pid = 1;
*/
static struct notifier_block *reboot_notifier_list;
-static DEFINE_RWLOCK(notifier_lock);
+
+/*
+ * Notifier chain core routines. The exported routines below
+ * are layered on top of these, with appropriate locking added.
+ */
+
+static int notifier_chain_register(struct notifier_block **nl,
+ struct notifier_block *n)
+{
+ while ((*nl) != NULL) {
+ if (n->priority > (*nl)->priority)
+ break;
+ nl = &((*nl)->next);
+ }
+ n->next = *nl;
+ rcu_assign_pointer(*nl, n);
+ return 0;
+}
+
+static int notifier_chain_unregister(struct notifier_block **nl,
+ struct notifier_block *n)
+{
+ while ((*nl) != NULL) {
+ if ((*nl) == n) {
+ rcu_assign_pointer(*nl, n->next);
+ return 0;
+ }
+ nl = &((*nl)->next);
+ }
+ return -ENOENT;
+}
+
+static int notifier_call_chain(struct notifier_block **nl,
+ unsigned long val, void *v)
+{
+ int ret = NOTIFY_DONE;
+ struct notifier_block *nb;
+
+ nb = rcu_dereference(*nl);
+ while (nb) {
+ ret = nb->notifier_call(nb, val, v);
+ if ((ret & NOTIFY_STOP_MASK) == NOTIFY_STOP_MASK)
+ break;
+ nb = rcu_dereference(nb->next);
+ }
+ return ret;
+}
+
+/*
+ * Atomic notifier chain routines. Registration and unregistration
+ * use a mutex, and call_chain is synchronized by RCU (no locks).
+ */
/**
- * notifier_chain_register - Add notifier to a notifier chain
- * @list: Pointer to root list pointer
+ * atomic_notifier_chain_register - Add notifier to an atomic notifier chain
+ * @nh: Pointer to head of the atomic notifier chain
* @n: New entry in notifier chain
*
- * Adds a notifier to a notifier chain.
+ * Adds a notifier to an atomic notifier chain.
+ * Must be called in process context.
*
* Currently always returns zero.
*/
+
+int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
+ struct notifier_block *n)
+{
+ int ret;
+
+ down(&nh->sem);
+ ret = notifier_chain_register(&nh->head, n);
+ up(&nh->sem);
+ return ret;
+}
+
+EXPORT_SYMBOL(atomic_notifier_chain_register);
+
+/**
+ * atomic_notifier_chain_unregister - Remove notifier from an atomic notifier chain
+ * @nh: Pointer to head of the atomic notifier chain
+ * @n: Entry to remove from notifier chain
+ *
+ * Removes a notifier from an atomic notifier chain.
+ * Must be called from process context.
+ *
+ * Returns zero on success, or %-ENOENT on failure.
+ */
+int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
+ struct notifier_block *n)
+{
+ int ret;
+
+ down(&nh->sem);
+ ret = notifier_chain_unregister(&nh->head, n);
+ up(&nh->sem);
+ synchronize_rcu();
+ return ret;
+}
+
+EXPORT_SYMBOL(atomic_notifier_chain_unregister);
+
+/**
+ * atomic_notifier_call_chain - Call functions in an atomic notifier chain
+ * @nh: Pointer to head of the atomic notifier chain
+ * @val: Value passed unmodified to notifier function
+ * @v: Pointer passed unmodified to notifier function
+ *
+ * Calls each function in a notifier chain in turn.
+ * This routine uses RCU to synchronize with changes in the chain.
+ *
+ * If the return value of the notifier can be and'd
+ * with %NOTIFY_STOP_MASK, then notifier_call_chain
+ * will return immediately, with the return value of
+ * the notifier function which halted execution.
+ * Otherwise, the return value is the return value
+ * of the last notifier function called.
+ */
-int notifier_chain_register(struct notifier_block **list, struct notifier_block *n)
+int atomic_notifier_call_chain(struct atomic_notifier_head *nh,
+ unsigned long val, void *v)
{
- write_lock(¬ifier_lock);
- while(*list)
- {
- if(n->priority > (*list)->priority)
- break;
- list= &((*list)->next);
- }
- n->next = *list;
- *list=n;
- write_unlock(¬ifier_lock);
- return 0;
+ int ret;
+
+ rcu_read_lock();
+ ret = notifier_call_chain(&nh->head, val, v);
+ rcu_read_unlock();
+ return ret;
}
-EXPORT_SYMBOL(notifier_chain_register);
+EXPORT_SYMBOL(atomic_notifier_call_chain);
+
+/*
+ * Blocking notifier chain routines. All access to the chain is
+ * synchronized by an rwsem.
+ */
/**
- * notifier_chain_unregister - Remove notifier from a notifier chain
- * @nl: Pointer to root list pointer
+ * blocking_notifier_chain_register - Add notifier to a blocking notifier chain
+ * @nh: Pointer to head of the blocking notifier chain
* @n: New entry in notifier chain
*
- * Removes a notifier from a notifier chain.
+ * Adds a notifier to a blocking notifier chain.
+ * Must be called in process context.
*
- * Returns zero on success, or %-ENOENT on failure.
+ * Currently always returns zero.
*/
-int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n)
+int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
+ struct notifier_block *n)
{
- write_lock(¬ifier_lock);
- while((*nl)!=NULL)
- {
- if((*nl)==n)
- {
- *nl=n->next;
- write_unlock(¬ifier_lock);
- return 0;
- }
- nl=&((*nl)->next);
- }
- write_unlock(¬ifier_lock);
- return -ENOENT;
+ int ret;
+
+ down_write(&nh->rwsem);
+ ret = notifier_chain_register(&nh->head, n);
+ up_write(&nh->rwsem);
+ return ret;
+}
+
+EXPORT_SYMBOL(blocking_notifier_chain_register);
+
+/**
+ * blocking_notifier_chain_unregister - Remove notifier from a blocking notifier chain
+ * @nh: Pointer to head of the blocking notifier chain
+ * @n: Entry to remove from notifier chain
+ *
+ * Removes a notifier from a blocking notifier chain.
+ * Must be called from process context.
+ *
+ * Returns zero on success, or %-ENOENT on failure.
+ */
+int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
+ struct notifier_block *n)
+{
+ int ret;
+
+ down_write(&nh->rwsem);
+ ret = notifier_chain_unregister(&nh->head, n);
+ up_write(&nh->rwsem);
+ return ret;
}
-EXPORT_SYMBOL(notifier_chain_unregister);
+EXPORT_SYMBOL(blocking_notifier_chain_unregister);
/**
- * notifier_call_chain - Call functions in a notifier chain
- * @n: Pointer to root pointer of notifier chain
+ * blocking_notifier_call_chain - Call functions in a blocking notifier chain
+ * @nh: Pointer to head of the blocking notifier chain
* @val: Value passed unmodified to notifier function
* @v: Pointer passed unmodified to notifier function
*
@@ -168,24 +293,85 @@ EXPORT_SYMBOL(notifier_chain_unregister)
* of the last notifier function called.
*/
-int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
+int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
+ unsigned long val, void *v)
{
- int ret=NOTIFY_DONE;
- struct notifier_block *nb = *n;
+ int ret;
- while(nb)
- {
- ret=nb->notifier_call(nb,val,v);
- if(ret&NOTIFY_STOP_MASK)
- {
- return ret;
- }
- nb=nb->next;
- }
+ down_read(&nh->rwsem);
+ ret = notifier_call_chain(&nh->head, val, v);
+ up_read(&nh->rwsem);
return ret;
}
-EXPORT_SYMBOL(notifier_call_chain);
+EXPORT_SYMBOL(blocking_notifier_call_chain);
+
+/*
+ * Raw notifier chain routines. There is no protection;
+ * the caller must provide it. Use at your own risk!
+ */
+
+/**
+ * raw_notifier_chain_register - Add notifier to a raw notifier chain
+ * @nh: Pointer to head of the raw notifier chain
+ * @n: New entry in notifier chain
+ *
+ * Adds a notifier to a raw notifier chain.
+ * All locking must be provided by the caller.
+ *
+ * Currently always returns zero.
+ */
+
+int raw_notifier_chain_register(struct raw_notifier_head *nh,
+ struct notifier_block *n)
+{
+ return notifier_chain_register(&nh->head);
+}
+
+EXPORT_SYMBOL(raw_notifier_chain_register);
+
+/**
+ * raw_notifier_chain_unregister - Remove notifier from a raw notifier chain
+ * @nh: Pointer to head of the raw notifier chain
+ * @n: Entry to remove from notifier chain
+ *
+ * Removes a notifier from a raw notifier chain.
+ * All locking must be provided by the caller.
+ *
+ * Returns zero on success, or %-ENOENT on failure.
+ */
+int raw_notifier_chain_unregister(struct raw_notifier_head *nh,
+ struct notifier_block *n)
+{
+ return notifier_chain_unregister(&nh->head, n);
+}
+
+EXPORT_SYMBOL(raw_notifier_chain_unregister);
+
+/**
+ * raw_notifier_call_chain - Call functions in a raw notifier chain
+ * @nh: Pointer to head of the raw notifier chain
+ * @val: Value passed unmodified to notifier function
+ * @v: Pointer passed unmodified to notifier function
+ *
+ * Calls each function in a notifier chain in turn.
+ * All locking must be provided by the caller.
+ *
+ * If the return value of the notifier can be and'd
+ * with %NOTIFY_STOP_MASK, then notifier_call_chain
+ * will return immediately, with the return value of
+ * the notifier function which halted execution.
+ * Otherwise, the return value is the return value
+ * of the last notifier function called.
+ */
+
+int raw_notifier_call_chain(struct raw_notifier_head *nh,
+ unsigned long val, void *v)
+{
+ return notifier_call_chain(&nh->head, val, v);
+}
+
+EXPORT_SYMBOL(raw_notifier_call_chain);
/**
* register_reboot_notifier - Register function to be called at reboot time
-
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