Use a non-sleeping lock to protect fmlp locking operations

The wait_queue_t lock used to protect fmlp_down and fmlp_up operations
in vanilla kernel is a sleeping lock in PreemptRT. We call fmlp
operations from atomic contexts. This commit replaces the wait_queue
lock with a raw_spin_lock.

2 files changed, 11 insertions, 5 deletions

diff --git a/include/litmus/sched_plugin.h b/include/litmus/sched_plugin.h
index 9c1c9f28ba79..c44108991053 100644
--- a/include/litmus/sched_plugin.h
+++ b/include/litmus/sched_plugin.h
@@ -11,6 +11,11 @@
 struct pi_semaphore {
         atomic_t count;
         int sleepers;
+        /* in PreemptRT the spinlock in the wait_queue_head_t
+         * is a sleeping spinlock; we use it in atomic, so a
+         * non-sleeping raw_spinlock_t is needed.
+         */
+        raw_spinlock_t  wait_lock;
         wait_queue_head_t wait;
         struct {
                 /* highest-prio holder/waiter */
diff --git a/litmus/fmlp.c b/litmus/fmlp.c
index 03fa7358d5eb..7f3ae0b3392a 100644
--- a/litmus/fmlp.c
+++ b/litmus/fmlp.c
@@ -30,6 +30,7 @@ static  void* create_fmlp_semaphore(void)
                 return NULL;
         atomic_set(&sem->count, 1);
         sem->sleepers = 0;
+        raw_spin_lock_init(&sem->wait_lock);
         init_waitqueue_head(&sem->wait);
         sem->hp.task = NULL;
         sem->holder = NULL;
@@ -142,7 +143,7 @@ static int do_fmlp_down(struct pi_semaphore* sem)
 
         pair.tsk = tsk;
         pair.sem = sem;
-        spin_lock_irqsave(&sem->wait.lock, flags);
+        raw_spin_lock_irqsave(&sem->wait_lock, flags);
 
         if (atomic_dec_return(&sem->count) < 0 ||
             waitqueue_active(&sem->wait)) {
@@ -154,7 +155,7 @@ static int do_fmlp_down(struct pi_semaphore* sem)
                 litmus->pi_block(sem, tsk);
 
                 /* release lock before sleeping */
-                spin_unlock_irqrestore(&sem->wait.lock, flags);
+                raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 
                 TS_PI_DOWN_END;
                 preempt_enable_no_resched();
@@ -186,7 +187,7 @@ static int do_fmlp_down(struct pi_semaphore* sem)
                 sem->hp.cpu_task[get_partition(tsk)] = tsk;
 
                 litmus->inherit_priority(sem, tsk);
-                spin_unlock_irqrestore(&sem->wait.lock, flags);
+                raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
         }
         return suspended;
 }
@@ -195,7 +196,7 @@ static void do_fmlp_up(struct pi_semaphore* sem)
 {
         unsigned long flags;
 
-        spin_lock_irqsave(&sem->wait.lock, flags);
+        raw_spin_lock_irqsave(&sem->wait_lock, flags);
 
         TRACE_CUR("releases PI lock %p\n", sem);
         litmus->return_priority(sem);
@@ -204,7 +205,7 @@ static void do_fmlp_up(struct pi_semaphore* sem)
                 /* there is a task queued */
                 wake_up_locked(&sem->wait);
 
-        spin_unlock_irqrestore(&sem->wait.lock, flags);
+        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 }
 
 asmlinkage long sys_fmlp_down(int sem_od)