MIPS: SMP: Fix possibility of deadlock when bringing CPUs online

This patch fixes the possibility of a deadlock when bringing up
secondary CPUs.
The deadlock occurs because the set_cpu_online() is called before
synchronise_count_slave(). This can cause a deadlock if the boot CPU,
having scheduled another thread, attempts to send an IPI to the
secondary CPU, which it sees has been marked online. The secondary is
blocked in synchronise_count_slave() waiting for the boot CPU to enter
synchronise_count_master(), but the boot cpu is blocked in
smp_call_function_many() waiting for the secondary to respond to it's
IPI request.

Fix this by marking the CPU online in cpu_callin_map and synchronising
counters before declaring the CPU online and calculating the maps for
IPIs.
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Reported-by: Justin Chen <justinpopo6@gmail.com>
Tested-by: Justin Chen <justinpopo6@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: stable@vger.kernel.org # v4.1+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14302/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smp.c

@@ -322,6 +322,9 @@ asmlinkage void start_secondary(void)
 	cpumask_set_cpu(cpu, &cpu_coherent_mask);
 	notify_cpu_starting(cpu);
 
+	cpumask_set_cpu(cpu, &cpu_callin_map);
+	synchronise_count_slave(cpu);
+
 	set_cpu_online(cpu, true);
 
 	set_cpu_sibling_map(cpu);
@@ -329,10 +332,6 @@ asmlinkage void start_secondary(void)
 
 	calculate_cpu_foreign_map();
 
-	cpumask_set_cpu(cpu, &cpu_callin_map);
-
-	synchronise_count_slave(cpu);
-
 	/*
 	 * irq will be enabled in ->smp_finish(), enabling it too early
 	 * is dangerous.