rcutorture: Dynamically allocate rcu_fwds structure

This commit switches from static structure to dynamic allocation
for rcu_fwds as another step towards providing multiple call_rcu()
forward-progress kthreads.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1686,7 +1686,7 @@ struct rcu_fwd {
 	unsigned long rcu_launder_gp_seq_start;
 };
 
-struct rcu_fwd rcu_fwds;
+struct rcu_fwd *rcu_fwds;
 bool rcu_fwd_emergency_stop;
 
 static void rcu_torture_fwd_cb_hist(struct rcu_fwd *rfp)
@@ -1952,7 +1952,7 @@ 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 = rcu_fwds;
 
 	WARN(1, "%s invoked upon OOM during forward-progress testing.\n",
 	     __func__);
@@ -2010,7 +2010,7 @@ static int rcu_torture_fwd_prog(void *args)
 /* If forward-progress checking is requested and feasible, spawn the thread. */
 static int __init rcu_torture_fwd_prog_init(void)
 {
-	struct rcu_fwd *rfp = &rcu_fwds;
+	struct rcu_fwd *rfp;
 
 	if (!fwd_progress)
 		return 0; /* Not requested, so don't do it. */
@@ -2026,12 +2026,15 @@ static int __init rcu_torture_fwd_prog_init(void)
 		WARN_ON(1); /* Make sure rcutorture notices conflict. */
 		return 0;
 	}
-	spin_lock_init(&rfp->rcu_fwd_lock);
-	rfp->rcu_fwd_cb_tail = &rfp->rcu_fwd_cb_head;
 	if (fwd_progress_holdoff <= 0)
 		fwd_progress_holdoff = 1;
 	if (fwd_progress_div <= 0)
 		fwd_progress_div = 4;
+	rfp = kzalloc(sizeof(*rfp), GFP_KERNEL);
+	if (!rfp)
+		return -ENOMEM;
+	spin_lock_init(&rfp->rcu_fwd_lock);
+	rfp->rcu_fwd_cb_tail = &rfp->rcu_fwd_cb_head;
 	return torture_create_kthread(rcu_torture_fwd_prog, rfp, fwd_prog_task);
 }