fork: Redo ifdefs around task stack handling

The use of ifdef CONFIG_VMAP_STACK is confusing in terms what is
actually happenning and what can happen.

For instance from reading free_thread_stack() it appears that in the
CONFIG_VMAP_STACK case it may receive a non-NULL vm pointer but it may also
be NULL in which case __free_pages() is used to free the stack.  This is
however not the case because in the VMAP case a non-NULL pointer is always
returned here.  Since it looks like this might happen, the compiler creates
the correct dead code with the invocation to __free_pages() and everything
around it. Twice.

Add spaces between the ifdef and the identifer to recognize the ifdef
level which is currently in scope.

Add the current identifer as a comment behind #else and #endif.
Move the code within free_thread_stack() and alloc_thread_stack_node()
into the relevant ifdef blocks.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220217102406.3697941-2-bigeasy@linutronix.de

kernel/fork.c

@@ -185,7 +185,7 @@ static inline void free_task_struct(struct task_struct *tsk)
  */
 # if THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK)
 
-#ifdef CONFIG_VMAP_STACK
+#  ifdef CONFIG_VMAP_STACK
 /*
  * vmalloc() is a bit slow, and calling vfree() enough times will force a TLB
  * flush.  Try to minimize the number of calls by caching stacks.
@@ -210,11 +210,9 @@ static int free_vm_stack_cache(unsigned int cpu)
 
 	return 0;
 }
-#endif
 
 static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 {
-#ifdef CONFIG_VMAP_STACK
 	void *stack;
 	int i;
 
@@ -258,45 +256,53 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 		tsk->stack = stack;
 	}
 	return stack;
-#else
-	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
-					     THREAD_SIZE_ORDER);
-
-	if (likely(page)) {
-		tsk->stack = kasan_reset_tag(page_address(page));
-		return tsk->stack;
-	}
-	return NULL;
-#endif
 }
 
-static inline void free_thread_stack(struct task_struct *tsk)
+static void free_thread_stack(struct task_struct *tsk)
 {
-#ifdef CONFIG_VMAP_STACK
 	struct vm_struct *vm = task_stack_vm_area(tsk);
+	int i;
 
-	if (vm) {
-		int i;
+	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+		memcg_kmem_uncharge_page(vm->pages[i], 0);
 
-		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
-			memcg_kmem_uncharge_page(vm->pages[i], 0);
+	for (i = 0; i < NR_CACHED_STACKS; i++) {
+		if (this_cpu_cmpxchg(cached_stacks[i], NULL,
+				     tsk->stack_vm_area) != NULL)
+			continue;
 
-		for (i = 0; i < NR_CACHED_STACKS; i++) {
-			if (this_cpu_cmpxchg(cached_stacks[i],
-					NULL, tsk->stack_vm_area) != NULL)
-				continue;
+		tsk->stack = NULL;
+		tsk->stack_vm_area = NULL;
+		return;
+	}
+	vfree_atomic(tsk->stack);
+	tsk->stack = NULL;
+	tsk->stack_vm_area = NULL;
+}
 
-			return;
-		}
+#  else /* !CONFIG_VMAP_STACK */
 
-		vfree_atomic(tsk->stack);
-		return;
+static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
+{
+	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
+					     THREAD_SIZE_ORDER);
+
+	if (likely(page)) {
+		tsk->stack = kasan_reset_tag(page_address(page));
+		return tsk->stack;
 	}
-#endif
+	return NULL;
+}
 
+static void free_thread_stack(struct task_struct *tsk)
+{
 	__free_pages(virt_to_page(tsk->stack), THREAD_SIZE_ORDER);
+	tsk->stack = NULL;
 }
-# else
+
+#  endif /* CONFIG_VMAP_STACK */
+# else /* !(THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK)) */
+
 static struct kmem_cache *thread_stack_cache;
 
 static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
@@ -312,6 +318,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
 static void free_thread_stack(struct task_struct *tsk)
 {
 	kmem_cache_free(thread_stack_cache, tsk->stack);
+	tsk->stack = NULL;
 }
 
 void thread_stack_cache_init(void)
@@ -321,8 +328,9 @@ void thread_stack_cache_init(void)
 					THREAD_SIZE, NULL);
 	BUG_ON(thread_stack_cache == NULL);
 }
-# endif
-#endif
+
+# endif /* THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK) */
+#endif /* !CONFIG_ARCH_THREAD_STACK_ALLOCATOR */
 
 /* SLAB cache for signal_struct structures (tsk->signal) */
 static struct kmem_cache *signal_cachep;
@@ -432,10 +440,6 @@ static void release_task_stack(struct task_struct *tsk)
 
 	account_kernel_stack(tsk, -1);
 	free_thread_stack(tsk);
-	tsk->stack = NULL;
-#ifdef CONFIG_VMAP_STACK
-	tsk->stack_vm_area = NULL;
-#endif
 }
 
 #ifdef CONFIG_THREAD_INFO_IN_TASK