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SubjectRe: [PATCH] Allow lockless traversal of notifier lists
On Mon, Nov 28, 2005 at 11:12:03PM +0530, Dipankar Sarma wrote:
> Don't we insert at the front of the list ? Shouldn't the read-side
> on alpha see the contents of the new notifier block before it sees
> the pointer to the first notifier block in the list head ?

Ok third version, hopefully Dipankar proof now.

Andrew, please consider applying.



As discussed in other thread.

Notifiers could be locklessly traversed if there was no removal
ever, except that the update order is wrong.

Just needed an additional write barrier, so that a parallel
running lockup can never see inconsistent state. As long as there
is no unregistration or the unregistration is done using
locking or RCU in the caller they should be ok now.

This only makes a difference on non i386/x86-64 architectures.
x86 was already ok because it never reorders writes.

Signed-off-by: Andi Kleen <>

diff -u linux-2.6.15rc2-work/kernel/sys.c-o linux-2.6.15rc2-work/kernel/sys.c
--- linux-2.6.15rc2-work/kernel/sys.c-o 2005-11-16 00:34:33.000000000 +0100
+++ linux-2.6.15rc2-work/kernel/sys.c 2005-11-29 00:33:26.000000000 +0100
@@ -102,6 +102,9 @@
* @n: New entry in notifier chain
* Adds a notifier to a notifier chain.
+ * As long as unregister is not used this is safe against parallel
+ * lockless notifier lookups. If unregister is used then unregister
+ * needs to do additional locking or use RCU.
* Currently always returns zero.
@@ -116,6 +119,7 @@
list= &((*list)->next);
n->next = *list;
+ smp_wmb();
return 0;
@@ -129,6 +133,8 @@
* @n: New entry in notifier chain
* Removes a notifier from a notifier chain.
+ * Note this needs additional locking or RCU in the caller to be safe
+ * against parallel traversals.
* Returns zero on success, or %-ENOENT on failure.
@@ -171,10 +177,12 @@
int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
int ret=NOTIFY_DONE;
- struct notifier_block *nb = *n;
+ struct notifier_block *nb;
+ smp_read_barrier_depends();
+ nb = *n;
+ smp_read_barrier_depends();
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