[patch] fix xenbus race

[Gerd Knorr <kraxel@suse.de>]

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   Hi,

Watch events may get lost, the xenstored response races against the new 
watch being added to the watch list in register_xenbus_watch().  Fixed 
by adding the watch to the list unconditionally, and remove it in the 
error case.

That one fixes the network issues I'm seeing.  Due to the race the 
"online" hotplug event for the vif backend was lost sometimes.

please apply,

   Gerd

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diff -r 43676a509982 linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_xs.c
--- a/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_xs.c	Sun Oct 30 18:23:36 2005
+++ b/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_xs.c	Mon Oct 31 15:08:14 2005
@@ -682,14 +682,15 @@
 
 	spin_lock(&watches_lock);
 	BUG_ON(find_watch(token));
+	list_add(&watch->list, &watches);
 	spin_unlock(&watches_lock);
 
 	err = xs_watch(watch->node, token);
 
 	/* Ignore errors due to multiple registration. */
-	if ((err == 0) || (err == -EEXIST)) {
+	if ((err != 0) && (err != -EEXIST)) {
 		spin_lock(&watches_lock);
-		list_add(&watch->list, &watches);
+		list_del(&watch->list);
 		spin_unlock(&watches_lock);
 	}
 

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