linux-kernel - [PATCH] locking/rwsem: Reinit wake

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Date:   Sat, 21 Jan 2017 21:33:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] locking/rwsem: Reinit wake_q after use

In __rwsem_down_write_failed_common(), the same wake_q variable name
is defined twice, with the inner wake_q hiding the one in outer scope.
We can either use different names for the two wake_q's. Even better,
we can use the same wake_q twice, if necessary. To enable the latter
change, we need to define a new helper function wake_q_init() to
enable reinitalization of wake_q after use.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 include/linux/sched.h       | 6 ++++++
 kernel/locking/rwsem-xadd.c | 7 +++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8a9bbb6..3640bde 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1019,6 +1019,12 @@ struct wake_q_head {
 #define DEFINE_WAKE_Q(name)				\
 	struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
 
+static inline void wake_q_init(struct wake_q_head *head)
+{
+	head->first = WAKE_Q_TAIL;
+	head->lastp = &head->first;
+}
+
 extern void wake_q_add(struct wake_q_head *head,
 		       struct task_struct *task);
 extern void wake_up_q(struct wake_q_head *head);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index a3a381a..2ad8d8d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -502,8 +502,6 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
 		 * wake any read locks that were queued ahead of us.
 		 */
 		if (count > RWSEM_WAITING_BIAS) {
-			DEFINE_WAKE_Q(wake_q);
-
 			__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
 			/*
 			 * The wakeup is normally called _after_ the wait_lock
@@ -513,6 +511,11 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
 			 * for attempting rwsem_try_write_lock().
 			 */
 			wake_up_q(&wake_q);
+
+			/*
+			 * Reinitialize wake_q after use.
+			 */
+			wake_q_init(&wake_q);
 		}
 
 	} else
-- 
1.8.3.1