[PATCH] task_work: avoid unneeded cmpxchg() in task_work_run()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

We only require cmpxchg()&retry when task is exiting.
xchg() is enough in other cases like original code in ac3d0da8.

So we try our best to use xchg() and avoid competition&latency
from task_work_add().

Also remove the inner loop

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 65bd3c9..82a42e7 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -56,14 +56,13 @@ void task_work_run(void)
 		 * work->func() can do task_work_add(), do not set
 		 * work_exited unless the list is empty.
 		 */
-		do {
-			work = ACCESS_ONCE(task->task_works);
-			head = !work && (task->flags & PF_EXITING) ?
-				&work_exited : NULL;
-		} while (cmpxchg(&task->task_works, work, head) != work);
-
-		if (!work)
+		if (!ACCESS_ONCE(task->task_works) ||
+		    !(work = xchg(&task->task_works, NULL))) {
+			if ((task->flags & PF_EXITING) &&
+			    cmpxchg(&task->task_works, NULL, &work_exited))
+				continue;
 			break;
+		}
 		/*
 		 * Synchronize with task_work_cancel(). It can't remove
 		 * the first entry == work, cmpxchg(task_works) should