linux-kernel - [PATCH tip/core/rcu 01/19] rcu: Stop disabling CPU hotplug in synchronize_rcu

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Date:	Fri, 17 Jul 2015 16:29:06 -0700
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 01/19] rcu: Stop disabling CPU hotplug in synchronize_rcu_expedited()

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

The fact that tasks could be migrated from leaf to root rcu_node
structures meant that synchronize_rcu_expedited() had to disable
CPU hotplug.  However, tasks now stay put, so this commit removes the
CPU-hotplug disabling from synchronize_rcu_expedited().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree_plugin.h | 25 ++-----------------------
 1 file changed, 2 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5dac0a10a985..7234f03e0aa2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -728,42 +728,23 @@ void synchronize_rcu_expedited(void)
 	smp_mb(); /* Above access cannot bleed into critical section. */
 
 	/*
-	 * Block CPU-hotplug operations.  This means that any CPU-hotplug
-	 * operation that finds an rcu_node structure with tasks in the
-	 * process of being boosted will know that all tasks blocking
-	 * this expedited grace period will already be in the process of
-	 * being boosted.  This simplifies the process of moving tasks
-	 * from leaf to root rcu_node structures.
-	 */
-	if (!try_get_online_cpus()) {
-		/* CPU-hotplug operation in flight, fall back to normal GP. */
-		wait_rcu_gp(call_rcu);
-		return;
-	}
-
-	/*
 	 * Acquire lock, falling back to synchronize_rcu() if too many
 	 * lock-acquisition failures.  Of course, if someone does the
 	 * expedited grace period for us, just leave.
 	 */
 	while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
 		if (ULONG_CMP_LT(snap,
-		    READ_ONCE(sync_rcu_preempt_exp_count))) {
-			put_online_cpus();
+		    READ_ONCE(sync_rcu_preempt_exp_count)))
 			goto mb_ret; /* Others did our work for us. */
-		}
 		if (trycount++ < 10) {
 			udelay(trycount * num_online_cpus());
 		} else {
-			put_online_cpus();
 			wait_rcu_gp(call_rcu);
 			return;
 		}
 	}
-	if (ULONG_CMP_LT(snap, READ_ONCE(sync_rcu_preempt_exp_count))) {
-		put_online_cpus();
+	if (ULONG_CMP_LT(snap, READ_ONCE(sync_rcu_preempt_exp_count)))
 		goto unlock_mb_ret; /* Others did our work for us. */
-	}
 
 	/* force all RCU readers onto ->blkd_tasks lists. */
 	synchronize_sched_expedited();
@@ -779,8 +760,6 @@ void synchronize_rcu_expedited(void)
 	rcu_for_each_leaf_node(rsp, rnp)
 		sync_rcu_preempt_exp_init2(rsp, rnp);
 
-	put_online_cpus();
-
 	/* Wait for snapshotted ->blkd_tasks lists to drain. */
 	rnp = rcu_get_root(rsp);
 	wait_event(sync_rcu_preempt_exp_wq,
-- 
1.8.1.5

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