[PATCH] locking/rwsem: Reinit wake_q after use

[PATCH] locking/rwsem: Reinit wake_q after use

[Waiman Long]

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>
---
 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

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

[tip-bot for Waiman Long]

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

[PATCH -tip] sched/wake_q: Clarify queue reinit

[Davidlohr Bueso]

As of bcc9a76d5ac (locking/rwsem: Reinit wake_q after use), the
comment regarding the list reinitialization no longer applies,
update it with the new wake_q_init() helper.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/linux/sched.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3640bde9f982..c0bc626c531f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -998,8 +998,8 @@ enum cpu_idle_type {
  *
  * The DEFINE_WAKE_Q macro declares and initializes the list head.
  * wake_up_q() does NOT reinitialize the list; it's expected to be
- * called near the end of a function, where the fact that the queue is
- * not used again will be easy to see by inspection.
+ * called near the end of a function. Otherwise, the list can be
+ * re-initialized for later re-use by wake_q_init().
  *
  * Note that this can cause spurious wakeups. schedule() callers
  * must ensure the call is done inside a loop, confirming that the
-- 
2.6.6

Re: [PATCH -tip] sched/wake_q: Clarify queue reinit

[Waiman Long]

On 01/29/2017 10:15 AM, Davidlohr Bueso wrote:
> As of bcc9a76d5ac (locking/rwsem: Reinit wake_q after use), the
> comment regarding the list reinitialization no longer applies,
> update it with the new wake_q_init() helper.
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
> include/linux/sched.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 3640bde9f982..c0bc626c531f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -998,8 +998,8 @@ enum cpu_idle_type {
>  *
>  * The DEFINE_WAKE_Q macro declares and initializes the list head.
>  * wake_up_q() does NOT reinitialize the list; it's expected to be
> - * called near the end of a function, where the fact that the queue is
> - * not used again will be easy to see by inspection.
> + * called near the end of a function. Otherwise, the list can be
> + * re-initialized for later re-use by wake_q_init().
>  *
>  * Note that this can cause spurious wakeups. schedule() callers
>  * must ensure the call is done inside a loop, confirming that the

Acked-by: Waiman Long <longman@redhat.com>

[tip:locking/core] sched/wake_q: Clarify queue reinit comment

[tip-bot for Davidlohr Bueso]

Commit-ID:  0754445d71c37a7afd4f0790a9be4cf53c1b8cc4
Gitweb:     http://git.kernel.org/tip/0754445d71c37a7afd4f0790a9be4cf53c1b8cc4
Author:     Davidlohr Bueso <dave@stgolabs.net>
AuthorDate: Sun, 29 Jan 2017 07:15:31 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 1 Feb 2017 10:02:18 +0100

sched/wake_q: Clarify queue reinit comment

As of:

  bcc9a76d5ac ("locking/rwsem: Reinit wake_q after use")

the comment regarding the list reinitialization no longer applies,
update it with the new wake_q_init() helper.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Waiman Long <longman@redhat.com>
Cc: peterz@infradead.org
Cc: longman@redhat.com
Cc: akpm@linux-foundation.org
Cc: paulmck@linux.vnet.ibm.com
Cc: torvalds@linux-foundation.org
Link: http://lkml.kernel.org/r/20170129151531.GA2444@linux-80c1.suse
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4e62b37..1cc0ded 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1000,8 +1000,8 @@ enum cpu_idle_type {
  *
  * The DEFINE_WAKE_Q macro declares and initializes the list head.
  * wake_up_q() does NOT reinitialize the list; it's expected to be
- * called near the end of a function, where the fact that the queue is
- * not used again will be easy to see by inspection.
+ * called near the end of a function. Otherwise, the list can be
+ * re-initialized for later re-use by wake_q_init().
  *
  * Note that this can cause spurious wakeups. schedule() callers
  * must ensure the call is done inside a loop, confirming that the