semaphore: signal safety

kernel-src/include/kernel/scheduler.h

@@ -121,7 +121,7 @@ void sched_queue(thread_t *thread);
 void sched_stopcurrentthread();
 int sched_yield();
 void sched_preparesleep(bool interruptible);
-void sched_wakeup(thread_t *thread, int reason);
+bool sched_wakeup(thread_t *thread, int reason);
 int sched_sleep();
 proc_t *sched_newproc();
 vnode_t *sched_getcwd();

kernel-src/lib/semaphore.c

@@ -17,7 +17,8 @@ static void insert(semaphore_t *sem, thread_t *thread) {
 
 static thread_t *get(semaphore_t *sem) {
 	thread_t *thread = sem->tail;
-	__assert(thread);
+	if (thread == NULL)
+		return NULL;
 
 	sem->tail = thread->sleepprev;
 
@@ -44,6 +45,11 @@ int semaphore_wait(semaphore_t *sem, bool interruptible) {
 		sched_preparesleep(interruptible);
 		spinlock_release(&sem->lock);
 		ret = sched_yield();
+		if (ret) {
+			spinlock_acquire(&sem->lock);
+			++sem->i;
+			spinlock_release(&sem->lock);
+		}
 		goto leave;
 	}
 
@@ -57,9 +63,17 @@ void semaphore_signal(semaphore_t *sem) {
 	bool intstate = interrupt_set(false);
 	spinlock_acquire(&sem->lock);
 	if (++sem->i <= 0) {
-		thread_t *thread = get(sem);
-		// XXX the current method doesn't respect thread priorities
-		sched_wakeup(thread, 0);
+		thread_t *thread;
+		do {
+			// wake SOMEONE up. this will *try* to wake up a thread
+			// it could wake up none though, as a thread could have been interrupted by a signal
+			thread = get(sem);
+			// XXX the current method doesn't respect thread priorities
+			if (thread) {
+				sched_wakeup(thread, 0);
+				break;
+			}
+		} while (thread);
 	}
 	spinlock_release(&sem->lock);
 	interrupt_set(intstate);

kernel-src/sys/scheduler.c

@@ -417,14 +417,14 @@ void sched_preparesleep(bool interruptible) {
 	_cpu()->thread->flags |= SCHED_THREAD_FLAGS_SLEEP | (interruptible ? SCHED_THREAD_FLAGS_INTERRUPTIBLE : 0);
 }
 
-void sched_wakeup(thread_t *thread, int reason) {
+bool sched_wakeup(thread_t *thread, int reason) {
 	bool intstate = interrupt_set(false);
 	spinlock_acquire(&thread->sleeplock);
 
 	if ((thread->flags & SCHED_THREAD_FLAGS_SLEEP) == 0 || ((reason == SCHED_WAKEUP_REASON_INTERRUPTED) && (thread->flags & SCHED_THREAD_FLAGS_INTERRUPTIBLE) == 0)) {
 		spinlock_release(&thread->sleeplock);
 		interrupt_set(intstate);
-		return;
+		return false;
 	}
 
 	thread->flags &= ~(SCHED_THREAD_FLAGS_SLEEP | SCHED_THREAD_FLAGS_INTERRUPTIBLE);
@@ -433,6 +433,8 @@ void sched_wakeup(thread_t *thread, int reason) {
 	spinlock_release(&thread->sleeplock);
 	sched_queue(thread);
 	interrupt_set(intstate);
+
+	return true;
 }
 
 static vnode_t *getnodeslock(vnode_t **addr) {