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

diff --git a/kernel/fork.c b/kernel/fork.c
index a024bf6..f5cc101 100644 (file)
--- a/kernel/fork.c
+++ b/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