Improve FMLP queue management.

The next owner of a FMLP-protected resource is dequeued from
the FMLP FIFO queue by unlock() (when the resource is freed by
the previous owner) instead of performing the dequeue by the next
owner immediately after it has been woken up.

This simplifies the code a little bit and also reduces potential
spinlock contention.

3 files changed, 9 insertions, 13 deletions

diff --git a/litmus/locking.c b/litmus/locking.c
index 728b56835cf7..2693f1aca859 100644
--- a/litmus/locking.c
+++ b/litmus/locking.c
@@ -107,16 +107,18 @@ asmlinkage long sys_litmus_unlock(int lock_od)
         return err;
 }
 
-struct task_struct* waitqueue_first(wait_queue_head_t *wq)
+struct task_struct* __waitqueue_remove_first(wait_queue_head_t *wq)
 {
-        wait_queue_t *q;
+        wait_queue_t* q;
+        struct task_struct* t = NULL;
 
         if (waitqueue_active(wq)) {
                 q = list_entry(wq->task_list.next,
                                wait_queue_t, task_list);
-                return (struct task_struct*) q->private;
-        } else
-                return NULL;
+                t = (struct task_struct*) q->private;
+                __remove_wait_queue(wq, q);
+        }
+        return(t);
 }
 
 
diff --git a/litmus/sched_gsn_edf.c b/litmus/sched_gsn_edf.c
index 770004544edd..dfec80e351aa 100644
--- a/litmus/sched_gsn_edf.c
+++ b/litmus/sched_gsn_edf.c
@@ -783,8 +783,6 @@ int gsnedf_fmlp_lock(struct litmus_lock* l)
                  * ->owner. We can thus check it without acquiring the spin
                  * lock. */
                 BUG_ON(sem->owner != t);
-
-                remove_wait_queue(&sem->wait, &wait);
         } else {
                 /* it's ours now */
                 sem->owner = t;
@@ -810,7 +808,7 @@ int gsnedf_fmlp_unlock(struct litmus_lock* l)
         }
 
         /* check if there are jobs waiting for this resource */
-        next = waitqueue_first(&sem->wait);
+        next = __waitqueue_remove_first(&sem->wait);
         if (next) {
                 /* next becomes the resouce holder */
                 sem->owner = next;
diff --git a/litmus/sched_psn_edf.c b/litmus/sched_psn_edf.c
index 9e62b0504b20..8e4a22dd8d6a 100644
--- a/litmus/sched_psn_edf.c
+++ b/litmus/sched_psn_edf.c
@@ -441,10 +441,6 @@ int psnedf_fmlp_lock(struct litmus_lock* l)
                  * ->owner. We can thus check it without acquiring the spin
                  * lock. */
                 BUG_ON(sem->owner != t);
-
-                /* FIXME: could we punt the dequeuing to the previous job,
-                 * which is holding the spinlock anyway? */
-                remove_wait_queue(&sem->wait, &wait);
         } else {
                 /* it's ours now */
                 sem->owner = t;
@@ -477,7 +473,7 @@ int psnedf_fmlp_unlock(struct litmus_lock* l)
         unboost_priority(t);
 
         /* check if there are jobs waiting for this resource */
-        next = waitqueue_first(&sem->wait);
+        next = __waitqueue_remove_first(&sem->wait);
         if (next) {
                 /* boost next job */
                 boost_priority(next);