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Subject[PATCH 2/4] rwsem: shorter spinlocked section in rwsem_down_failed_common()
Date
This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
sections in rwsem_down_failed_common():

- We only need the sem->wait_lock to insert ourselves on the wait_list;
the waiter node can be prepared outside of the wait_lock.

- The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
before testing waiter.task to see if someone woke us; it doesn't need to
protect the entire function.

Signed-off-by: Michel Lespinasse <walken@google.com>

---
lib/rwsem.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 4a6ff093a433..cd2d803cbbe4 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -180,14 +180,12 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
struct task_struct *tsk = current;
signed long count;

- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
-
/* set up my own style of waitqueue */
- raw_spin_lock_irq(&sem->wait_lock);
waiter.task = tsk;
waiter.type = type;
get_task_struct(tsk);

+ raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list))
adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter.list, &sem->wait_list);
@@ -210,11 +208,11 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
raw_spin_unlock_irq(&sem->wait_lock);

/* wait to be given the lock */
- for (;;) {
+ while (true) {
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!waiter.task)
break;
schedule();
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}

tsk->state = TASK_RUNNING;
--
1.8.1

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