[PATCH-tip v7 09/20] locking/rwsem: Always release wait_lock before waking up tasks

[Waiman Long]

With the use of wake_q, we can do task wakeups without holding the
wait_lock. There is one exception in the rwsem code, though. It is
when the writer in the slowpath detects that there are waiters ahead
but the rwsem is not held by a writer. This can lead to a long wait_lock
hold time especially when a large number of readers are to be woken up.

Remediate this situation by releasing the wait_lock before waking up
tasks and re-acquiring it afterward.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/linux/sched/wake_q.h |  5 +++++
 kernel/locking/rwsem.c       | 30 +++++++++++++++++++-----------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
index ad826d2a4557..26a2013ac39c 100644
--- a/include/linux/sched/wake_q.h
+++ b/include/linux/sched/wake_q.h
@@ -51,6 +51,11 @@ static inline void wake_q_init(struct wake_q_head *head)
 	head->lastp = &head->first;
 }
 
+static inline bool wake_q_empty(struct wake_q_head *head)
+{
+	return head->first == WAKE_Q_TAIL;
+}
+
 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);
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index dd90884a80b8..750f407b83cf 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -747,17 +747,25 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 					? RWSEM_WAKE_READERS
 					: RWSEM_WAKE_ANY, &wake_q);
 
-		/*
-		 * The wakeup is normally called _after_ the wait_lock
-		 * is released, but given that we are proactively waking
-		 * readers we can deal with the wake_q overhead as it is
-		 * similar to releasing and taking the wait_lock again
-		 * for attempting rwsem_try_write_lock().
-		 */
-		wake_up_q(&wake_q);
-
-		/* We need wake_q again below, reinitialize */
-		wake_q_init(&wake_q);
+		if (!wake_q_empty(&wake_q)) {
+			/*
+			 * We want to minimize wait_lock hold time especially
+			 * when a large number of readers are to be woken up.
+			 */
+			raw_spin_unlock_irq(&sem->wait_lock);
+			wake_up_q(&wake_q);
+			wake_q_init(&wake_q);	/* Used again, reinit */
+			raw_spin_lock_irq(&sem->wait_lock);
+			/*
+			 * This waiter may have become first in the wait
+			 * list after re-acquring the wait_lock. The
+			 * rwsem_first_waiter() test in the main while
+			 * loop below will correctly detect that. We do
+			 * need to reload count to perform proper trylock
+			 * and avoid missed wakeup.
+			 */
+			count = atomic_long_read(&sem->count);
+		}
 	} else {
 		count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
 	}
-- 
2.18.1