linux-kernel - [PATCH tip/core/rcu 04/13] rcu: Remove lock-acquisition loop from rcu_read_unlock

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Date:	Fri,  4 Dec 2015 16:10:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 04/13] 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@...ux.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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