[PATCH 02/10] perf: Avoid race between cpu hotplug and installing event

["Yan, Zheng" <zheng.z.yan@intel.com>]

From: "Yan, Zheng" <zheng.z.yan@intel.com>

perf_event_open requires the cpu on which to install event is online,
but the cpu can go offline after perf_event_open checks that. Add
get_online_cpus()/put_online_cpus() pair to avoid the race.

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
---
 kernel/events/core.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 91a4459..9fdf394 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6255,6 +6255,8 @@ SYSCALL_DEFINE5(perf_event_open,
 		}
 	}
 
+	get_online_cpus();
+
 	event = perf_event_alloc(&attr, cpu, task, group_leader, NULL,
 				 NULL, NULL);
 	if (IS_ERR(event)) {
@@ -6394,6 +6396,8 @@ SYSCALL_DEFINE5(perf_event_open,
 	perf_unpin_context(ctx);
 	mutex_unlock(&ctx->mutex);
 
+	put_online_cpus();
+
 	event->owner = current;
 
 	mutex_lock(&current->perf_event_mutex);
@@ -6422,6 +6426,7 @@ SYSCALL_DEFINE5(perf_event_open,
 err_alloc:
 	free_event(event);
 err_task:
+	put_online_cpus();
 	if (task)
 		put_task_struct(task);
 err_group_fd:
-- 
1.7.7.6