[RFC PATCH] epoll: Add synchronous wakeup support for ep_poll_callback

[Xuewen Yan]

Now, the epoll only use wake_up() interface to wake up task.
However, sometimes, there are epoll users which want to use
the synchronous wakeup flag to hint the scheduler, such as
Android binder driver.
So add a wake_up_sync() define, and use the wake_up_sync()
when the sync is true in ep_poll_callback().

Co-developed-by: Jing Xia <jing.xia@xxxxxxxxxx>
Signed-off-by: Jing Xia <jing.xia@xxxxxxxxxx>
Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
---
 fs/eventpoll.c       | 5 ++++-
 include/linux/wait.h | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 882b89edc52a..9b815e0a1ac5 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1336,7 +1336,10 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 				break;
 			}
 		}
-		wake_up(&ep->wq);
+		if (sync)
+			wake_up_sync(&ep->wq);
+		else
+			wake_up(&ep->wq);
 	}
 	if (waitqueue_active(&ep->poll_wait))
 		pwake++;
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 8aa3372f21a0..2b322a9b88a2 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -221,6 +221,7 @@ void __wake_up_pollfree(struct wait_queue_head *wq_head);
 #define wake_up_all(x)			__wake_up(x, TASK_NORMAL, 0, NULL)
 #define wake_up_locked(x)		__wake_up_locked((x), TASK_NORMAL, 1)
 #define wake_up_all_locked(x)		__wake_up_locked((x), TASK_NORMAL, 0)
+#define wake_up_sync(x)			__wake_up_sync(x, TASK_NORMAL)
 
 #define wake_up_interruptible(x)	__wake_up(x, TASK_INTERRUPTIBLE, 1, NULL)
 #define wake_up_interruptible_nr(x, nr)	__wake_up(x, TASK_INTERRUPTIBLE, nr, NULL)
-- 
2.25.1