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Subject[PATCH v5 36/44] tty: Inline ldsem down_failed() into down_{read,write}_failed()
Date
Separate the read and write lock paths to simplify handling of
initial acquire failure.

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

Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldsem.c | 70 ++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 57 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index a60d7e3..d849fb85 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -220,12 +220,14 @@ static void ldsem_wake(struct ld_semaphore *sem)
}

/*
- * wait for a lock to be granted
+ * wait for the read lock to be granted
*/
static struct ld_semaphore __sched *
-down_failed(struct ld_semaphore *sem, unsigned flags, long adjust, long timeout)
+down_read_failed(struct ld_semaphore *sem, long timeout)
{
struct ldsem_waiter waiter;
+ long flags = LDSEM_READ_WAIT;
+ long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;

/* set up my own style of waitqueue */
raw_spin_lock_irq(&sem->wait_lock);
@@ -279,22 +281,64 @@ down_failed(struct ld_semaphore *sem, unsigned flags, long adjust, long timeout)
}

/*
- * wait for the read lock to be granted
- */
-static struct ld_semaphore __sched *
-down_read_failed(struct ld_semaphore *sem, long timeout)
-{
- return down_failed(sem, LDSEM_READ_WAIT,
- -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS, timeout);
-}
-
-/*
* wait for the write lock to be granted
*/
static struct ld_semaphore __sched *
down_write_failed(struct ld_semaphore *sem, long timeout)
{
- return down_failed(sem, LDSEM_WRITE_WAIT, -LDSEM_ACTIVE_BIAS, timeout);
+ struct ldsem_waiter waiter;
+ long flags = LDSEM_WRITE_WAIT;
+ long adjust = -LDSEM_ACTIVE_BIAS;
+
+ /* set up my own style of waitqueue */
+ raw_spin_lock_irq(&sem->wait_lock);
+
+ if (flags & LDSEM_READ_WAIT)
+ list_add_tail(&waiter.list, &sem->read_wait);
+ else
+ list_add_tail(&waiter.list, &sem->write_wait);
+
+ waiter.task = current;
+ waiter.flags = flags;
+ get_task_struct(current);
+
+ /* change the lock attempt to a wait --
+ * if there are no active locks, wake the new lock owner(s)
+ */
+ if ((ldsem_atomic_update(adjust, sem) & LDSEM_ACTIVE_MASK) == 0)
+ __ldsem_wake(sem, LDSEM_WAKE_NO_CHECK);
+
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ /* wait to be given the lock */
+ for (;;) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+
+ if (!waiter.task)
+ break;
+ if (!timeout)
+ break;
+ timeout = schedule_timeout(timeout);
+ }
+
+ __set_current_state(TASK_RUNNING);
+
+ if (!timeout) {
+ /* lock timed out but check if this task was just
+ * granted lock ownership - if so, pretend there
+ * was no timeout; otherwise, cleanup lock wait */
+ raw_spin_lock_irq(&sem->wait_lock);
+ if (waiter.task) {
+ ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
+ list_del(&waiter.list);
+ put_task_struct(waiter.task);
+ raw_spin_unlock_irq(&sem->wait_lock);
+ return NULL;
+ }
+ raw_spin_unlock_irq(&sem->wait_lock);
+ }
+
+ return sem;
}


--
1.8.1.2


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