Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 17, 2018 at 12:53:10PM -0800, Davidlohr Bueso wrote:
> On Mon, 17 Dec 2018, Peter Zijlstra wrote:
> > 
> > I've put some patches here:
> > 
> >  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/core
> > 
> > Could you have a look?
> 
> So how about the following to reduce some of the performance penalty (at
> the cost of more complexity)?

I'd rather do it like so, except I'm still conflicted on the naming.

diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
index 545f37138057..ad826d2a4557 100644
--- a/include/linux/sched/wake_q.h
+++ b/include/linux/sched/wake_q.h
@@ -51,8 +51,8 @@ static inline void wake_q_init(struct wake_q_head *head)
 	head->lastp = &head->first;
 }
 
-extern void wake_q_add(struct wake_q_head *head,
-		       struct task_struct *task);
+extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
+extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
 extern void wake_up_q(struct wake_q_head *head);
 
 #endif /* _LINUX_SCHED_WAKE_Q_H */
diff --git a/kernel/futex.c b/kernel/futex.c
index d14971f6ed3d..6218d98f649b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1402,8 +1402,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
 	 * Queue the task for later wakeup for after we've released
 	 * the hb->lock. wake_q_add() grabs reference to p.
 	 */
-	wake_q_add(wake_q, p);
-	put_task_struct(p);
+	wake_q_add_safe(wake_q, p);
 }
 
 /*
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 50d9af615dc4..fbe96341beee 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -211,9 +211,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 		 * 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);
+		wake_q_add_safe(wake_q, tsk);
 	}
 
 	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d740d7a3608d..72d82ce73714 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -408,7 +408,7 @@ static bool set_nr_if_polling(struct task_struct *p)
  * This function must be used as-if it were wake_up_process(); IOW the task
  * must be ready to be woken at this location.
  */
-void wake_q_add(struct wake_q_head *head, struct task_struct *task)
+static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
 {
 	struct wake_q_node *node = &task->wake_q;
 
@@ -422,15 +422,27 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
 	 */
 	smp_mb__before_atomic();
 	if (unlikely(cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)))
-		return;
-
-	get_task_struct(task);
+		return false;
 
 	/*
 	 * The head is context local, there can be no concurrency.
 	 */
 	*head->lastp = node;
 	head->lastp = &node->next;
+
+	return true;
+}
+
+void wake_q_add(struct wake_q_head *head, struct task_struct *task)
+{
+	if (__wake_q_add(head, task))
+		get_task_struct(task);
+}
+
+void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task)
+{
+	if (!__wake_a_add(head, task))
+		put_task_struct(task);
 }
 
 void wake_up_q(struct wake_q_head *head)