From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: linux-kernel@vger.kernel.org
Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 01/19] rcu: Stop disabling CPU hotplug in synchronize_rcu_expedited()
Date: Fri, 17 Jul 2015 16:29:06 -0700
Message-Id: <1437175764-24096-1-git-send-email-paulmck@linux.vnet.ibm.com>
In-Reply-To: <20150717232901.GA22511@linux.vnet.ibm.com>
References: <20150717232901.GA22511@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2746
Lines: 78

From: "Paul E. McKenney" <paulmck@linux.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@linux.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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/