[tip:locking/core] locking/rwsem: Fix (possible) missed wakeup

[tip-bot for Xie Yongji <tipbot@zytor.com>]

Commit-ID:  e158488be27b157802753a59b336142dc0eb0380
Gitweb:     https://git.kernel.org/tip/e158488be27b157802753a59b336142dc0eb0380
Author:     Xie Yongji <xieyongji@baidu.com>
AuthorDate: Thu, 29 Nov 2018 20:50:30 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 21 Jan 2019 11:15:39 +0100

locking/rwsem: Fix (possible) missed wakeup

Because wake_q_add() can imply an immediate wakeup (cmpxchg failure
case), we must not rely on the wakeup being delayed. However, commit:

  e38513905eea ("locking/rwsem: Rework zeroing reader waiter->task")

relies on exactly that behaviour in that the wakeup must not happen
until after we clear waiter->task.

[ peterz: Added changelog. ]

Signed-off-by: Xie Yongji <xieyongji@baidu.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: e38513905eea ("locking/rwsem: Rework zeroing reader waiter->task")
Link: https://lkml.kernel.org/r/1543495830-2644-1-git-send-email-xieyongji@baidu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/rwsem-xadd.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 09b180063ee1..50d9af615dc4 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -198,15 +198,22 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 		woken++;
 		tsk = waiter->task;
 
-		wake_q_add(wake_q, tsk);
+		get_task_struct(tsk);
 		list_del(&waiter->list);
 		/*
-		 * Ensure that the last operation is setting the reader
+		 * Ensure calling get_task_struct() before setting the reader
 		 * waiter to nil such that rwsem_down_read_failed() cannot
 		 * race with do_exit() by always holding a reference count
 		 * to the task to wakeup.
 		 */
 		smp_store_release(&waiter->task, NULL);
+		/*
+		 * Ensure issuing the wakeup (either by us or someone else)
+		 * after setting the reader waiter to nil.
+		 */
+		wake_q_add(wake_q, tsk);
+		/* wake_q_add() already take the task ref */
+		put_task_struct(tsk);
 	}
 
 	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;