rcu: Convert rcu_state.ofl_lock to raw_spinlock_t

1e64b15a4b10 ("rcu: Fix grace-period hangs due to race with CPU offline")
added spinlock_t ofl_lock to the rcu_state structure, then takes it with
preemption disabled during CPU offline, which gives the -rt patchset's
sleeping spinlock heartburn.

This commit therefore converts ->ofl_lock to raw_spinlock_t.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

kernel/rcu/tree.c

@@ -97,7 +97,7 @@ struct rcu_state rcu_state = {
 	.abbr = RCU_ABBR,
 	.exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex),
 	.exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex),
-	.ofl_lock = __SPIN_LOCK_UNLOCKED(rcu_state.ofl_lock),
+	.ofl_lock = __RAW_SPIN_LOCK_UNLOCKED(rcu_state.ofl_lock),
 };
 
 /* Dump rcu_node combining tree at boot to verify correct setup. */
@@ -1776,13 +1776,13 @@ static bool rcu_gp_init(void)
 	 */
 	rcu_state.gp_state = RCU_GP_ONOFF;
 	rcu_for_each_leaf_node(rnp) {
-		spin_lock(&rcu_state.ofl_lock);
+		raw_spin_lock(&rcu_state.ofl_lock);
 		raw_spin_lock_irq_rcu_node(rnp);
 		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
 		    !rnp->wait_blkd_tasks) {
 			/* Nothing to do on this leaf rcu_node structure. */
 			raw_spin_unlock_irq_rcu_node(rnp);
-			spin_unlock(&rcu_state.ofl_lock);
+			raw_spin_unlock(&rcu_state.ofl_lock);
 			continue;
 		}
 
@@ -1818,7 +1818,7 @@ static bool rcu_gp_init(void)
 		}
 
 		raw_spin_unlock_irq_rcu_node(rnp);
-		spin_unlock(&rcu_state.ofl_lock);
+		raw_spin_unlock(&rcu_state.ofl_lock);
 	}
 	rcu_gp_slow(gp_preinit_delay); /* Races with CPU hotplug. */
 
@@ -3377,7 +3377,7 @@ void rcu_report_dead(unsigned int cpu)
 
 	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
 	mask = rdp->grpmask;
-	spin_lock(&rcu_state.ofl_lock);
+	raw_spin_lock(&rcu_state.ofl_lock);
 	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
 	rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq);
 	rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags);
@@ -3388,7 +3388,7 @@ void rcu_report_dead(unsigned int cpu)
 	}
 	rnp->qsmaskinitnext &= ~mask;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-	spin_unlock(&rcu_state.ofl_lock);
+	raw_spin_unlock(&rcu_state.ofl_lock);
 
 	per_cpu(rcu_cpu_started, cpu) = 0;
 }

kernel/rcu/tree.h

@@ -343,7 +343,7 @@ struct rcu_state {
 	const char *name;			/* Name of structure. */
 	char abbr;				/* Abbreviated name. */
 
-	spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
+	raw_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
 						/* Synchronize offline with */
 						/*  GP pre-initialization. */
 };