rcutorture: Add grace period after CPU offline

Beyond a certain point in the CPU-hotplug offline process, timers get
stranded on the outgoing CPU, and won't fire until that CPU comes back
online, which might well be never.  This commit therefore adds a hook
in torture_onoff_init() that is invoked from torture_offline(), which
rcutorture uses to occasionally wait for a grace period.  This should
result in failures for RCU implementations that rely on stranded timers
eventually firing in the absence of the CPU coming back online.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

4 files changed, 18 insertions, 4 deletions

diff --git a/include/linux/torture.h b/include/linux/torture.h
index 48fad21109fc..f2d3bcbf4337 100644
--- a/include/linux/torture.h
+++ b/include/linux/torture.h
@@ -50,11 +50,12 @@
 	do { if (verbose) pr_alert("%s" TORTURE_FLAG "!!! %s\n", torture_type, s); } while (0)
 
 /* Definitions for online/offline exerciser. */
+typedef void torture_ofl_func(void);
 bool torture_offline(int cpu, long *n_onl_attempts, long *n_onl_successes,
 		     unsigned long *sum_offl, int *min_onl, int *max_onl);
 bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
 		    unsigned long *sum_onl, int *min_onl, int *max_onl);
-int torture_onoff_init(long ooholdoff, long oointerval);
+int torture_onoff_init(long ooholdoff, long oointerval, torture_ofl_func *f);
 void torture_onoff_stats(void);
 bool torture_onoff_failures(void);
 
diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index 7d0b0ed74404..c8b348097bb5 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -970,7 +970,7 @@ static int __init lock_torture_init(void)
 	/* Prepare torture context. */
 	if (onoff_interval > 0) {
 		firsterr = torture_onoff_init(onoff_holdoff * HZ,
-					      onoff_interval * HZ);
+					      onoff_interval * HZ, NULL);
 		if (firsterr)
 			goto unwind;
 	}
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 0955f3a20952..9eb9235c1ec9 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2243,6 +2243,14 @@ static void rcu_test_debug_objects(void)
 #endif /* #else #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 }
 
+static void rcutorture_sync(void)
+{
+	static unsigned long n;
+
+	if (cur_ops->sync && !(++n & 0xfff))
+		cur_ops->sync();
+}
+
 static int __init
 rcu_torture_init(void)
 {
@@ -2404,7 +2412,8 @@ rcu_torture_init(void)
 	firsterr = torture_shutdown_init(shutdown_secs, rcu_torture_cleanup);
 	if (firsterr)
 		goto unwind;
-	firsterr = torture_onoff_init(onoff_holdoff * HZ, onoff_interval);
+	firsterr = torture_onoff_init(onoff_holdoff * HZ, onoff_interval,
+				      rcutorture_sync);
 	if (firsterr)
 		goto unwind;
 	firsterr = rcu_torture_stall_init();
diff --git a/kernel/torture.c b/kernel/torture.c
index bbf6d473e50c..a03ff722352b 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -75,6 +75,7 @@ static DEFINE_MUTEX(fullstop_mutex);
 static struct task_struct *onoff_task;
 static long onoff_holdoff;
 static long onoff_interval;
+static torture_ofl_func *onoff_f;
 static long n_offline_attempts;
 static long n_offline_successes;
 static unsigned long sum_offline;
@@ -118,6 +119,8 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
 			pr_alert("%s" TORTURE_FLAG
 				 "torture_onoff task: offlined %d\n",
 				 torture_type, cpu);
+		if (onoff_f)
+			onoff_f();
 		(*n_offl_successes)++;
 		delta = jiffies - starttime;
 		*sum_offl += delta;
@@ -243,11 +246,12 @@ stop:
 /*
  * Initiate online-offline handling.
  */
-int torture_onoff_init(long ooholdoff, long oointerval)
+int torture_onoff_init(long ooholdoff, long oointerval, torture_ofl_func *f)
 {
 #ifdef CONFIG_HOTPLUG_CPU
 	onoff_holdoff = ooholdoff;
 	onoff_interval = oointerval;
+	onoff_f = f;
 	if (onoff_interval <= 0)
 		return 0;
 	return torture_create_kthread(torture_onoff, NULL, onoff_task);