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>
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 9eb7fed..a84dee2 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -52,8 +52,21 @@
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 @@
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)