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

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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@in.ibm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.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;