[PATCH 12/12] closures: fix a race on wakeup from closure_sync

[Kent Overstreet <kent.overstreet@xxxxxxxxx>]

Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
---
 lib/closure.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/closure.c b/lib/closure.c
index 46cfe4c382..3e6366c262 100644
--- a/lib/closure.c
+++ b/lib/closure.c
@@ -104,8 +104,14 @@ struct closure_syncer {
 
 static void closure_sync_fn(struct closure *cl)
 {
-	cl->s->done = 1;
-	wake_up_process(cl->s->task);
+	struct closure_syncer *s = cl->s;
+	struct task_struct *p;
+
+	rcu_read_lock();
+	p = READ_ONCE(s->task);
+	s->done = 1;
+	wake_up_process(p);
+	rcu_read_unlock();
 }
 
 void __sched __closure_sync(struct closure *cl)
-- 
2.20.1