[PATCH v2 18/45] softirq: Use get/put_online_cpus_atomic() to prevent CPU offline

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.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Frederic Weisbecker <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Sedat Dilek <redacted>
Cc: "Paul E. McKenney" <redacted>
Signed-off-by: Srivatsa S. Bhat <redacted>
---

 kernel/softirq.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 3d6833f..c289722 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c

@@ -644,14 +644,17 @@ static void remote_softirq_receive(void *data)
 
 static int __try_remote_softirq(struct call_single_data *cp, int cpu, int softirq)
 {
+	get_online_cpus_atomic();
 	if (cpu_online(cpu)) {
 		cp->func = remote_softirq_receive;
 		cp->info = &softirq;
 		cp->flags = 0;
 
 		__smp_call_function_single(cpu, cp, 0);
+		put_online_cpus_atomic();
 		return 0;
 	}
+	put_online_cpus_atomic();
 	return 1;
 }
 #else /* CONFIG_USE_GENERIC_SMP_HELPERS */