[PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU

[Paul E. McKenney]

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().

Reported-by: Peng Fan <van.freenix@xxxxxxxxx>
Reported-by: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Tested-by: Tested-by: Fabio Estevam <fabio.estevam@xxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b4fbf00ee4ad..75f85e20aafa 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -267,6 +267,14 @@ void __cpu_die(unsigned int cpu)
 }
 
 /*
+ * 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 @@ void arch_cpu_idle_dead(void)
 	/*
 	 * 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