ARM: CPU hotplug: Delegate complete() to surviving CPU

The ARM implementation of arch_cpu_idle_dead() invokes complete(), but
does so after RCU has stopped watching the outgoing CPU, which results
in lockdep complaints because complete() invokes functions containing RCU
readers.  This patch therefore uses Thomas Gleixner's trick of delegating
the complete() call to a surviving CPU via smp_call_function_single().

This patch is untested, and probably does not even build.

Reported by many people, but most recently by:

Reported-by: Peng Fan <van.freenix@gmail.com>
Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b4fbf00..75f85e2 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -267,6 +267,14 @@
 }
 
 /*
+ * Invoke complete() on behalf of the outgoing CPU.
+ */
+static void arch_cpu_idle_dead_complete(void *arg)
+{
+	complete(&cpu_died);
+}
+
+/*
  * Called from the idle thread for the CPU which has been shutdown.
  *
  * Note that we disable IRQs here, but do not re-enable them
@@ -293,9 +301,11 @@
 	/*
 	 * Tell __cpu_die() that this CPU is now safe to dispose of.  Once
 	 * this returns, power and/or clocks can be removed at any point
-	 * from this CPU and its cache by platform_cpu_kill().
+	 * from this CPU and its cache by platform_cpu_kill().  We cannot
+	 * call complete() this late, so we delegate it to an online CPU.
 	 */
-	complete(&cpu_died);
+	smp_call_function_single(cpumask_first(cpu_online_mask),
+				 arch_cpu_idle_dead_complete, NULL, 0);
 
 	/*
 	 * Ensure that the cache lines associated with that completion are