kern: gracefully handle resource shortage

* kern/thread.c (stack_alloc): Report resource shortage.
* kern/sched_prim.h (stack_alloc): Adjust declaration accordingly.
* kern/thread_swap.c (thread_doswapin): Report resource shortage.
(swapin_thread_continue): If the swap-in fails, put the thread back on
the queue and go back to sleep.
* kern/thread_swap.h (thread_doswapin): Adjust declaration accordingly.

4 files changed, 22 insertions, 10 deletions

diff --git a/kern/sched_prim.h b/kern/sched_prim.h
index fd989b6c..62698dc2 100644
--- a/kern/sched_prim.h
+++ b/kern/sched_prim.h
@@ -150,7 +150,7 @@ extern void	stack_handoff(
  *	or are defined directly by machine-dependent code.
  */
 
-extern void	stack_alloc(
+extern kern_return_t	stack_alloc(
 	thread_t	thread,
 	void		(*resume)(thread_t));
 extern boolean_t stack_alloc_try(
diff --git a/kern/thread.c b/kern/thread.c
index 009884ce..f52c95b8 100644
--- a/kern/thread.c
+++ b/kern/thread.c
@@ -171,7 +171,7 @@ boolean_t stack_alloc_try(
  *	May block.
  */
 
-void stack_alloc(
+kern_return_t stack_alloc(
 	thread_t	thread,
 	void		(*resume)(thread_t))
 {
@@ -195,15 +195,15 @@ void stack_alloc(
 	(void) splx(s);
 
 	if (stack == 0) {
+		kern_return_t kr;
 		/*
 		 *	Kernel stacks should be naturally aligned,
 		 *	so that it is easy to find the starting/ending
 		 *	addresses of a stack given an address in the middle.
 		 */
-
-		if (kmem_alloc_aligned(kmem_map, &stack, KERNEL_STACK_SIZE)
-							!= KERN_SUCCESS)
-			panic("stack_alloc");
+		kr = kmem_alloc_aligned(kmem_map, &stack, KERNEL_STACK_SIZE);
+		if (kr != KERN_SUCCESS)
+			return kr;
 
 #if	MACH_DEBUG
 		stack_init(stack);
@@ -211,6 +211,7 @@ void stack_alloc(
 	}
 
 	stack_attach(thread, stack, resume);
+	return KERN_SUCCESS;
 }
 
 /*
diff --git a/kern/thread_swap.c b/kern/thread_swap.c
index dc2924a9..20ad0409 100644
--- a/kern/thread_swap.c
+++ b/kern/thread_swap.c
@@ -123,15 +123,18 @@ void thread_swapin(thread_t thread)
  *	it on a run queue.  No locks should be held on entry, as it is
  *	likely that this routine will sleep (waiting for stack allocation).
  */
-void thread_doswapin(thread_t thread)
+kern_return_t thread_doswapin(thread_t thread)
 {
+	kern_return_t kr;
 	spl_t	s;
 
 	/*
 	 *	Allocate the kernel stack.
 	 */
 
-	stack_alloc(thread, thread_continue);
+	kr = stack_alloc(thread, thread_continue);
+	if (kr != KERN_SUCCESS)
+		return kr;
 
 	/*
 	 *	Place on run queue.  
@@ -144,6 +147,7 @@ void thread_doswapin(thread_t thread)
 		thread_setrun(thread, TRUE);
 	thread_unlock(thread);
 	(void) splx(s);
+	return KERN_SUCCESS;
 }
 
 /*
@@ -163,13 +167,20 @@ void __attribute__((noreturn)) swapin_thread_continue(void)
 
 		while ((thread = (thread_t) dequeue_head(&swapin_queue))
 							!= THREAD_NULL) {
+			kern_return_t kr;
 			swapper_unlock();
 			(void) splx(s);
 
-			thread_doswapin(thread);		/* may block */
+			kr = thread_doswapin(thread);		/* may block */
 
 			s = splsched();
 			swapper_lock();
+
+			if (kr != KERN_SUCCESS) {
+				enqueue_head(&swapin_queue,
+					     (queue_entry_t) thread);
+				break;
+			}
 		}
 
 		assert_wait((event_t) &swapin_queue, FALSE);
diff --git a/kern/thread_swap.h b/kern/thread_swap.h
index 9d645373..d032accf 100644
--- a/kern/thread_swap.h
+++ b/kern/thread_swap.h
@@ -37,7 +37,7 @@
  */
 extern void	swapper_init(void);
 extern void	thread_swapin(thread_t thread);
-extern void	thread_doswapin(thread_t thread);
+extern kern_return_t	thread_doswapin(thread_t thread);
 extern void	swapin_thread(void) __attribute__((noreturn));
 
 #endif	/* _KERN_THREAD_SWAP_H_ */