rcu: Improve grace-period start logic

This commit improves grace-period start logic by checking ->gp_flags
under the lock and by issuing a warning if a grace period is already
in progress.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 32618b3fe4e6aa375b3cd3bde1acd12bdd4237b3..d679a522c0a23e1f6fb3d6a608fe1f7aef51156c 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1297,7 +1297,7 @@ static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
 }
 
 /*
- * Initialize a new grace period.
+ * Initialize a new grace period.  Return 0 if no grace period required.
  */
 static int rcu_gp_init(struct rcu_state *rsp)
 {
@@ -1306,10 +1306,18 @@ static int rcu_gp_init(struct rcu_state *rsp)
 
        rcu_bind_gp_kthread();
        raw_spin_lock_irq(&rnp->lock);
+       if (rsp->gp_flags == 0) {
+               /* Spurious wakeup, tell caller to go back to sleep.  */
+               raw_spin_unlock_irq(&rnp->lock);
+               return 0;
+       }
        rsp->gp_flags = 0; /* Clear all flags: New grace period. */
 
-       if (rcu_gp_in_progress(rsp)) {
-               /* Grace period already in progress, don't start another.  */
+       if (WARN_ON_ONCE(rcu_gp_in_progress(rsp))) {
+               /*
+                * Grace period already in progress, don't start another.
+                * Not supposed to be able to happen.
+                */
                raw_spin_unlock_irq(&rnp->lock);
                return 0;
        }
@@ -1474,8 +1482,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                        wait_event_interruptible(rsp->gp_wq,
                                                 rsp->gp_flags &
                                                 RCU_GP_FLAG_INIT);
-                       if ((rsp->gp_flags & RCU_GP_FLAG_INIT) &&
-                           rcu_gp_init(rsp))
+                       if (rcu_gp_init(rsp))
                                break;
                        cond_resched();
                        flush_signals(current);