[PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
while being protected by a spinlock. Use the get/put_online_cpus_atomic()
APIs to prevent CPUs from going offline, while invoking from atomic context.

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

 kernel/rcutree.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index cf3adc6..caeed1a 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2107,6 +2107,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 			rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
 			continue;
 		}
+
+		get_online_cpus_atomic();
 		cpu = rnp->grplo;
 		bit = 1;
 		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
@@ -2114,6 +2116,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 			    f(per_cpu_ptr(rsp->rda, cpu)))
 				mask |= bit;
 		}
+		put_online_cpus_atomic();
+
 		if (mask != 0) {
 
 			/* rcu_report_qs_rnp() releases rnp->lock. */