rcu: Remove lock-acquisition loop from rcu_read_unlock_special()

Several releases have come and gone without the warning triggering,
so remove the lock-acquisition loop.  Retain the WARN_ON_ONCE()
out of sheer paranoia.

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

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 57ba873..ae4ce2b 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -449,19 +449,13 @@ void rcu_read_unlock_special(struct task_struct *t)
 
                /*
                 * Remove this task from the list it blocked on.  The task
-                * now remains queued on the rcu_node corresponding to
-                * the CPU it first blocked on, so the first attempt to
-                * acquire the task's rcu_node's ->lock will succeed.
-                * Keep the loop and add a WARN_ON() out of sheer paranoia.
+                * now remains queued on the rcu_node corresponding to the
+                * CPU it first blocked on, so there is no longer any need
+                * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
                 */
-               for (;;) {
-                       rnp = t->rcu_blocked_node;
-                       raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
-                       if (rnp == t->rcu_blocked_node)
-                               break;
-                       WARN_ON_ONCE(1);
-                       raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
-               }
+               rnp = t->rcu_blocked_node;
+               raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
+               WARN_ON_ONCE(rnp != t->rcu_blocked_node);
                empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
                empty_exp = sync_rcu_preempt_exp_done(rnp);
                smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */