smp_mb() (to make the memory operations visible globally) is needed before
sending the ipi, if the receiver refers to the data(setup by the sender) in
a lock-free fashion.

On x86, x2apic mode accesses for sending IPI's don't have serializing
semantics. So the need for smp_mb() before sending the IPI becomes more
critical in x2apic mode.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

Index: linux-2.6.git/kernel/smp.c
===================================================================
--- linux-2.6.git.orig/kernel/smp.c	2008-10-31 11:15:11.000000000 -0700
+++ linux-2.6.git/kernel/smp.c	2008-11-04 09:54:29.000000000 -0800
@@ -76,6 +76,11 @@
 	list_add_tail(&data->list, &dst->list);
 	spin_unlock_irqrestore(&dst->lock, flags);
 
+	/*
+	 * Make the list addition visible before sending the ipi.
+	 */
+	smp_mb();
+
 	if (ipi)
 		arch_send_call_function_single_ipi(cpu);
 
@@ -370,6 +375,11 @@
 	list_add_tail_rcu(&data->csd.list, &call_function_queue);
 	spin_unlock_irqrestore(&call_function_lock, flags);
 
+	/*
+	 * Make the list addition visible before sending the ipi.
+	 */
+	smp_mb();
+
 	/* Send a message to all CPUs in the map */
 	arch_send_call_function_ipi(mask);
 

-- 

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