[PATCH v5 39/44] tty: Factor ldsem writer trylock

[Peter Hurley]

Prepare to simplify lock acquistion by waiting writer.

Derived from Michel Lespinasse's write lock stealing work on rwsem.

Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/tty_ldsem.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index d2f091a..48f1ce8 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -136,13 +136,8 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)
 	sem->wait_readers = 0;
 }
 
-static void __ldsem_wake_writer(struct ld_semaphore *sem)
+static inline int writer_trylock(struct ld_semaphore *sem)
 {
-	struct ldsem_waiter *waiter;
-	struct task_struct *tsk;
-
-	waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
-
 	/* only wake this writer if the active part of the count can be
 	 * transitioned from 0 -> 1
 	 */
@@ -159,9 +154,22 @@ static void __ldsem_wake_writer(struct ld_semaphore *sem)
 		 */
 		count = ldsem_atomic_update(-LDSEM_ACTIVE_BIAS, sem);
 		if (count & LDSEM_ACTIVE_MASK)
-			return;
+			return 0;
 	} while (1);
 
+	return 1;
+}
+
+static void __ldsem_wake_writer(struct ld_semaphore *sem)
+{
+	struct ldsem_waiter *waiter;
+	struct task_struct *tsk;
+
+	waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
+
+	if (!writer_trylock(sem))
+		return;
+
 	/* We must be careful not to touch 'waiter' after we set ->task = NULL.
 	 * It is an allocated on the waiter's stack and may become invalid at
 	 * any time after that point (due to a wakeup from another source).
-- 
1.8.1.2

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