rcu: Prepare for synchronization fixes: clean up for non-NO_HZ handling of ->completed counter

Impose a clear locking design on non-NO_HZ handling of the
->completed counter.  This increases the distance between the
RCU and the CPU-hotplug mechanisms.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
Cc: <stable@kernel.org> # .32.x
LKML-Reference: <12571987491353-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 30 insertions, 37 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 69f4efec3449..26249abf24dc 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -178,9 +178,29 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
         return &rsp->node[0];
 }
 
+/*
+ * Record the specified "completed" value, which is later used to validate
+ * dynticks counter manipulations and CPU-offline checks.  Specify
+ * "rsp->completed - 1" to unconditionally invalidate any future dynticks
+ * manipulations and CPU-offline checks.  Such invalidation is useful at
+ * the beginning of a grace period.
+ */
+static void dyntick_record_completed(struct rcu_state *rsp, long comp)
+{
+        rsp->dynticks_completed = comp;
+}
+
 #ifdef CONFIG_SMP
 
 /*
+ * Recall the previously recorded value of the completion for dynticks.
+ */
+static long dyntick_recall_completed(struct rcu_state *rsp)
+{
+        return rsp->dynticks_completed;
+}
+
+/*
  * If the specified CPU is offline, tell the caller that it is in
  * a quiescent state.  Otherwise, whack it with a reschedule IPI.
  * Grace periods can end up waiting on an offline CPU when that
@@ -337,28 +357,9 @@ void rcu_irq_exit(void)
                 set_need_resched();
 }
 
-/*
- * Record the specified "completed" value, which is later used to validate
- * dynticks counter manipulations.  Specify "rsp->completed - 1" to
- * unconditionally invalidate any future dynticks manipulations (which is
- * useful at the beginning of a grace period).
- */
-static void dyntick_record_completed(struct rcu_state *rsp, long comp)
-{
-        rsp->dynticks_completed = comp;
-}
-
 #ifdef CONFIG_SMP
 
 /*
- * Recall the previously recorded value of the completion for dynticks.
- */
-static long dyntick_recall_completed(struct rcu_state *rsp)
-{
-        return rsp->dynticks_completed;
-}
-
-/*
  * Snapshot the specified CPU's dynticks counter so that we can later
  * credit them with an implicit quiescent state.  Return 1 if this CPU
  * is in dynticks idle mode, which is an extended quiescent state.
@@ -421,24 +422,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 
 #else /* #ifdef CONFIG_NO_HZ */
 
-static void dyntick_record_completed(struct rcu_state *rsp, long comp)
-{
-}
-
 #ifdef CONFIG_SMP
 
-/*
- * If there are no dynticks, then the only way that a CPU can passively
- * be in a quiescent state is to be offline.  Unlike dynticks idle, which
- * is a point in time during the prior (already finished) grace period,
- * an offline CPU is always in a quiescent state, and thus can be
- * unconditionally applied.  So just return the current value of completed.
- */
-static long dyntick_recall_completed(struct rcu_state *rsp)
-{
-        return rsp->completed;
-}
-
 static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
         return 0;
@@ -1146,6 +1131,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
         long lastcomp;
         struct rcu_node *rnp = rcu_get_root(rsp);
         u8 signaled;
+        u8 forcenow;
 
         if (!rcu_gp_in_progress(rsp))
                 return;  /* No grace period in progress, nothing to force. */
@@ -1182,16 +1168,23 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
                 if (rcu_process_dyntick(rsp, lastcomp,
                                         dyntick_save_progress_counter))
                         goto unlock_ret;
+                /* fall into next case. */
+
+        case RCU_SAVE_COMPLETED:
 
                 /* Update state, record completion counter. */
+                forcenow = 0;
                 spin_lock(&rnp->lock);
                 if (lastcomp == rsp->completed &&
-                    rsp->signaled == RCU_SAVE_DYNTICK) {
+                    rsp->signaled == signaled) {
                         rsp->signaled = RCU_FORCE_QS;
                         dyntick_record_completed(rsp, lastcomp);
+                        forcenow = signaled == RCU_SAVE_COMPLETED;
                 }
                 spin_unlock(&rnp->lock);
-                break;
+                if (!forcenow)
+                        break;
+                /* fall into next case. */
 
         case RCU_FORCE_QS: