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[Xen-devel] [patch] fix xenbus race

To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] [patch] fix xenbus race
From: Gerd Knorr <kraxel@xxxxxxx>
Date: Mon, 31 Oct 2005 15:16:05 +0100
Delivery-date: Mon, 31 Oct 2005 14:13:15 +0000
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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
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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