[PATCH kvm-unit-tests v2 2/2] arm/arm64: smp: add deadlock detection

[Andrew Jones <drjones@redhat.com>]

on_cpu() and friends are a bit risky when implemented without IPIs
(no preemption), because we can easily get deadlocks. Luckily, it's
also easy to detect those deadlocks, and assert, to at least make
them easier to debug.

Signed-off-by: Andrew Jones <drjones@redhat.com>
---
 lib/arm/smp.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/lib/arm/smp.c b/lib/arm/smp.c
index b4b43237e32e..bb999243de63 100644
--- a/lib/arm/smp.c
+++ b/lib/arm/smp.c
@@ -76,9 +76,24 @@ typedef void (*on_cpu_func)(void *);
 struct on_cpu_info {
 	on_cpu_func func;
 	void *data;
+	cpumask_t waiters;
 };
 static struct on_cpu_info on_cpu_info[NR_CPUS];
 
+static void cpu_wait(int cpu)
+{
+	int me = smp_processor_id();
+
+	if (cpu == me)
+		return;
+
+	cpumask_set_cpu(me, &on_cpu_info[cpu].waiters);
+	assert_msg(!cpumask_test_cpu(cpu, &on_cpu_info[me].waiters), "CPU%d <=> CPU%d deadlock detected", me, cpu);
+	while (!cpu_idle(cpu))
+		wfe();
+	cpumask_clear_cpu(me, &on_cpu_info[cpu].waiters);
+}
+
 void do_idle(void)
 {
 	int cpu = smp_processor_id();
@@ -117,8 +132,7 @@ void on_cpu_async(int cpu, void (*func)(void *data), void *data)
 	spin_unlock(&lock);
 
 	for (;;) {
-		while (!cpu_idle(cpu))
-			wfe();
+		cpu_wait(cpu);
 		spin_lock(&lock);
 		if ((volatile void *)on_cpu_info[cpu].func == NULL)
 			break;
@@ -134,9 +148,7 @@ void on_cpu_async(int cpu, void (*func)(void *data), void *data)
 void on_cpu(int cpu, void (*func)(void *data), void *data)
 {
 	on_cpu_async(cpu, func, data);
-
-	while (!cpu_idle(cpu))
-		wfe();
+	cpu_wait(cpu);
 }
 
 void on_cpus(void (*func)(void))
@@ -150,6 +162,14 @@ void on_cpus(void (*func)(void))
 	}
 	func();
 
+	for_each_present_cpu(cpu) {
+		if (cpu == me)
+			continue;
+		cpumask_set_cpu(me, &on_cpu_info[cpu].waiters);
+		assert_msg(!cpumask_test_cpu(cpu, &on_cpu_info[me].waiters), "CPU%d <=> CPU%d deadlock detected", me, cpu);
+	}
 	while (cpumask_weight(&cpu_idle_mask) < nr_cpus - 1)
 		wfe();
+	for_each_present_cpu(cpu)
+		cpumask_clear_cpu(me, &on_cpu_info[cpu].waiters);
 }
-- 
2.9.4