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>

4 files changed, 17 insertions, 12 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3fe03151a8e6..855fd0d3f174 100644
--- a/include/linux/sched.h
+++ b/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 */
 
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d903e2f2b840..71bc79791cd9 100644
--- a/kernel/rcutree.c
+++ b/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 ||
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index ca560364d8cd..bf8a6f9f134d 100644
--- a/kernel/rcutree.h
+++ b/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;
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 04343bee646d..47789369ea59 100644
--- a/kernel/rcutree_plugin.h
+++ b/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