netdev - [PATCH v3 16/45] rcu: Use cpu_is_offline_nocheck() to avoid false-positive warnings

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Message-ID: <20130627195517.29830.64108.stgit@srivatsabhat.in.ibm.com>
Date:	Fri, 28 Jun 2013 01:25:17 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org,
	laijs@...fujitsu.com, David.Laight@...lab.com
Cc:	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	srivatsa.bhat@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH v3 16/45] rcu: Use cpu_is_offline_nocheck() to avoid
 false-positive warnings

In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
while being protected by a spinlock. At first, it appears as if we need to
use the get/put_online_cpus_atomic() APIs to properly synchronize with CPU
hotplug, once we get rid of stop_machine(). However, RCU has adequate
synchronization with CPU hotplug, making that unnecessary. But since the
locking details are non-trivial, it is hard to teach this to the rudimentary
hotplug locking validator.

So use the _nocheck() variants of the cpu accessor functions to prevent false-
positive warnings from the CPU hotplug debug code. Also, add a comment
explaining the hotplug synchronization design used in RCU, so that its easy
to see why it is justified to use the _nocheck() variants.

Cc: Dipankar Sarma <dipankar@...ibm.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 kernel/rcutree.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index cf3adc6..ced28a45 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -794,7 +794,17 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
 		return 0;  /* Grace period is not old enough. */
 	barrier();
-	if (cpu_is_offline(rdp->cpu)) {
+
+	/*
+	 * It is safe to use the _nocheck() version of cpu_is_offline() here
+	 * (to avoid false-positive warnings from CPU hotplug debug code),
+	 * because:
+	 * 1. rcu_gp_init() holds off CPU hotplug operations during grace
+	 *    period initialization.
+	 * 2. The current grace period has not ended yet.
+	 * So it is safe to sample the offline state without synchronization.
+	 */
+	if (cpu_is_offline_nocheck(rdp->cpu)) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
 		rdp->offline_fqs++;
 		return 1;

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