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[PATCH v2 tip/core/rcu 04/14] rcu: Remove lock-acquisition loop from rcu_read_unlock_special()

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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>
---
 kernel/rcu/tree_plugin.h | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 57ba873d2f18..ae4ce2b665f8 100644
--- 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. */
-- 
2.5.2

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[PATCH v2 tip/core/rcu 0/14] Miscellaneous fixes for 4.5 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:10 +0100
  [PATCH v2 tip/core/rcu 14/14] list: Add lockless list traversal primitives "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 12/14] rcu: Move wakeup out from under rnp->lock "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 02/14] kernel: Make rcu/tree_trace.c explicitly non-modular "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 09/14] rcu: Make cpu_needs_another_gp() be bool "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 04/14] rcu: Remove lock-acquisition loop from rcu_read_unlock_special() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 06/14] rcu: Avoid tick_nohz_active checks on NOCBs CPUs "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 07/14] rcu: Stop disabling interrupts in scheduler fastpaths "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 10/14] rcu: Don't redundantly disable irqs in rcu_irq_{enter,exit}() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:20 +0100
  [PATCH v2 tip/core/rcu 03/14] rcu: Simplify rcu_sched_qs() control flow "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100
  [PATCH v2 tip/core/rcu 01/14] rcu: Move lock_class_key to local scope "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100
  [PATCH v2 tip/core/rcu 11/14] rcu: Fix comment for rcu_dereference_raw_notrace "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100
  [PATCH v2 tip/core/rcu 13/14] rcu: Make rcu_gp_init() be bool rather than int "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100
  [PATCH v2 tip/core/rcu 08/14] rcu: Eliminate unused rcu_init_one() argument "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100
  [PATCH v2 tip/core/rcu 05/14] rcu: Fix obsolete rcu_bootup_announce_oddness() comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-12-10 00:30 +0100

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