rcutorture: Properly synchronize with OOM notifier

The current rcutorture forward-progress code assumes that it is the
only cause of out-of-memory (OOM) events.  For script-based rcutorture
testing, this assumption is in fact correct.  However, testing based
on modprobe/rmmod might well encounter external OOM events, which could
happen at any time.

This commit therefore properly synchronizes the interaction between
rcutorture's forward-progress testing and its OOM notifier by adding a
global mutex.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1796,6 +1796,7 @@ struct rcu_fwd {
 	unsigned long rcu_launder_gp_seq_start;
 };
 
+static DEFINE_MUTEX(rcu_fwd_mutex);
 static struct rcu_fwd *rcu_fwds;
 static bool rcu_fwd_emergency_stop;
 
@@ -2062,8 +2063,14 @@ static void rcu_torture_fwd_prog_cr(struct rcu_fwd *rfp)
 static int rcutorture_oom_notify(struct notifier_block *self,
 				 unsigned long notused, void *nfreed)
 {
-	struct rcu_fwd *rfp = rcu_fwds;
+	struct rcu_fwd *rfp;
 
+	mutex_lock(&rcu_fwd_mutex);
+	rfp = rcu_fwds;
+	if (!rfp) {
+		mutex_unlock(&rcu_fwd_mutex);
+		return NOTIFY_OK;
+	}
 	WARN(1, "%s invoked upon OOM during forward-progress testing.\n",
 	     __func__);
 	rcu_torture_fwd_cb_hist(rfp);
@@ -2081,6 +2088,7 @@ static int rcutorture_oom_notify(struct notifier_block *self,
 	smp_mb(); /* Frees before return to avoid redoing OOM. */
 	(*(unsigned long *)nfreed)++; /* Forward progress CBs freed! */
 	pr_info("%s returning after OOM processing.\n", __func__);
+	mutex_unlock(&rcu_fwd_mutex);
 	return NOTIFY_OK;
 }
 
@@ -2148,7 +2156,9 @@ static int __init rcu_torture_fwd_prog_init(void)
 		return -ENOMEM;
 	spin_lock_init(&rfp->rcu_fwd_lock);
 	rfp->rcu_fwd_cb_tail = &rfp->rcu_fwd_cb_head;
+	mutex_lock(&rcu_fwd_mutex);
 	rcu_fwds = rfp;
+	mutex_unlock(&rcu_fwd_mutex);
 	return torture_create_kthread(rcu_torture_fwd_prog, rfp, fwd_prog_task);
 }
 
@@ -2158,7 +2168,9 @@ static void rcu_torture_fwd_prog_cleanup(void)
 
 	torture_stop_kthread(rcu_torture_fwd_prog, fwd_prog_task);
 	rfp = rcu_fwds;
+	mutex_lock(&rcu_fwd_mutex);
 	rcu_fwds = NULL;
+	mutex_unlock(&rcu_fwd_mutex);
 	kfree(rfp);
 }