[tip:locking/core] locking/rwsem: Reinit wake_q after use

[tip-bot for Waiman Long <tipbot@zytor.com>]

Commit-ID:  bcc9a76d5ac426bc45c9e863b1830347827ca77a
Gitweb:     http://git.kernel.org/tip/bcc9a76d5ac426bc45c9e863b1830347827ca77a
Author:     Waiman Long <longman@redhat.com>
AuthorDate: Sat, 21 Jan 2017 21:33:35 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sun, 22 Jan 2017 09:54:00 +0100

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@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1485052415-9611-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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 f4f9d32..4e62b37 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1021,6 +1021,12 @@ 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 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
 		 * 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 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
 			 * for attempting rwsem_try_write_lock().
 			 */
 			wake_up_q(&wake_q);
+
+			/*
+			 * Reinitialize wake_q after use.
+			 */
+			wake_q_init(&wake_q);
 		}
 
 	} else