rcu: Changes from reviews: avoid casts, fix/add warnings, improve comments

Changes suggested by review comments from Josh Triplett and
Mathieu Desnoyers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <20090827220012.GA30525@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1163,6 +1163,8 @@ struct sched_rt_entity {
 #endif
 };
 
+struct rcu_node;
+
 struct task_struct {
 	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
 	void *stack;
@@ -1208,7 +1210,7 @@ struct task_struct {
 #ifdef CONFIG_TREE_PREEMPT_RCU
 	int rcu_read_lock_nesting;
 	char rcu_read_unlock_special;
-	void *rcu_blocked_node;
+	struct rcu_node *rcu_blocked_node;
 	struct list_head rcu_node_entry;
 #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
 

kernel/rcutree.c

@@ -229,7 +229,6 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
 #endif /* #ifdef CONFIG_SMP */
 
 #ifdef CONFIG_NO_HZ
-static DEFINE_RATELIMIT_STATE(rcu_rs, 10 * HZ, 5);
 
 /**
  * rcu_enter_nohz - inform RCU that current CPU is entering nohz
@@ -249,7 +248,7 @@ void rcu_enter_nohz(void)
 	rdtp = &__get_cpu_var(rcu_dynticks);
 	rdtp->dynticks++;
 	rdtp->dynticks_nesting--;
-	WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
+	WARN_ON_ONCE(rdtp->dynticks & 0x1);
 	local_irq_restore(flags);
 }
 
@@ -268,7 +267,7 @@ void rcu_exit_nohz(void)
 	rdtp = &__get_cpu_var(rcu_dynticks);
 	rdtp->dynticks++;
 	rdtp->dynticks_nesting++;
-	WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs);
+	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
 	local_irq_restore(flags);
 	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
@@ -287,7 +286,7 @@ void rcu_nmi_enter(void)
 	if (rdtp->dynticks & 0x1)
 		return;
 	rdtp->dynticks_nmi++;
-	WARN_ON_RATELIMIT(!(rdtp->dynticks_nmi & 0x1), &rcu_rs);
+	WARN_ON_ONCE(!(rdtp->dynticks_nmi & 0x1));
 	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
 
@@ -306,7 +305,7 @@ void rcu_nmi_exit(void)
 		return;
 	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
 	rdtp->dynticks_nmi++;
-	WARN_ON_RATELIMIT(rdtp->dynticks_nmi & 0x1, &rcu_rs);
+	WARN_ON_ONCE(rdtp->dynticks_nmi & 0x1);
 }
 
 /**
@@ -322,7 +321,7 @@ void rcu_irq_enter(void)
 	if (rdtp->dynticks_nesting++)
 		return;
 	rdtp->dynticks++;
-	WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs);
+	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
 	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
 
@@ -341,7 +340,7 @@ void rcu_irq_exit(void)
 		return;
 	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
 	rdtp->dynticks++;
-	WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
+	WARN_ON_ONCE(rdtp->dynticks & 0x1);
 
 	/* If the interrupt queued a callback, get out of dyntick mode. */
 	if (__get_cpu_var(rcu_sched_data).nxtlist ||

kernel/rcutree.h

@@ -81,6 +81,8 @@ struct rcu_dynticks {
 struct rcu_node {
 	spinlock_t lock;
 	long	gpnum;		/* Current grace period for this node. */
+				/*  This will either be equal to or one */
+				/*  behind the root rcu_node's gpnum. */
 	unsigned long qsmask;	/* CPUs or groups that need to switch in */
 				/*  order for current grace period to proceed.*/
 	unsigned long qsmaskinit;

kernel/rcutree_plugin.h

@@ -92,7 +92,7 @@ static void rcu_preempt_qs(int cpu)
 		rnp = rdp->mynode;
 		spin_lock(&rnp->lock);
 		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
-		t->rcu_blocked_node = (void *)rnp;
+		t->rcu_blocked_node = rnp;
 
 		/*
 		 * If this CPU has already checked in, then this task
@@ -176,9 +176,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		 * most one time.  So at most two passes through loop.
 		 */
 		for (;;) {
-			rnp = (struct rcu_node *)t->rcu_blocked_node;
+			rnp = t->rcu_blocked_node;
 			spin_lock(&rnp->lock);
-			if (rnp == (struct rcu_node *)t->rcu_blocked_node)
+			if (rnp == t->rcu_blocked_node)
 				break;
 			spin_unlock(&rnp->lock);
 		}
@@ -288,8 +288,10 @@ static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
 	struct rcu_node *rnp_root = rcu_get_root(rsp);
 	struct task_struct *tp;
 
-	if (rnp == rnp_root)
+	if (rnp == rnp_root) {
+		WARN_ONCE(1, "Last CPU thought to be offlined?");
 		return;  /* Shouldn't happen: at least one CPU online. */
+	}
 
 	/*
 	 * Move tasks up to root rcu_node.  Rely on the fact that the