kthread: introduce to_live_kthread()

"k->vfork_done != NULL" with a barrier() after to_kthread(k) in
task_get_live_kthread(k) looks unclear, and sub-optimal because we load
->vfork_done twice.

All we need is to ensure that we do not return to_kthread(NULL).  Add a
new trivial helper which loads/checks ->vfork_done once, this also looks
more understandable.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 10 deletions

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 9eb7fed0bbaa..a84dee265f94 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -52,8 +52,21 @@ enum KTHREAD_BITS {
         KTHREAD_IS_PARKED,
 };
 
-#define to_kthread(tsk) \
-        container_of((tsk)->vfork_done, struct kthread, exited)
+#define __to_kthread(vfork)     \
+        container_of(vfork, struct kthread, exited)
+
+static inline struct kthread *to_kthread(struct task_struct *k)
+{
+        return __to_kthread(k->vfork_done);
+}
+
+static struct kthread *to_live_kthread(struct task_struct *k)
+{
+        struct completion *vfork = ACCESS_ONCE(k->vfork_done);
+        if (likely(vfork))
+                return __to_kthread(vfork);
+        return NULL;
+}
 
 /**
  * kthread_should_stop - should this kthread return now?
@@ -313,15 +326,8 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 
 static struct kthread *task_get_live_kthread(struct task_struct *k)
 {
-        struct kthread *kthread;
-
         get_task_struct(k);
-        kthread = to_kthread(k);
-        /* It might have exited */
-        barrier();
-        if (k->vfork_done != NULL)
-                return kthread;
-        return NULL;
+        return to_live_kthread(k);
 }
 
 static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)