by tip-bot2 for Kyle Huey

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Subject: [tip: core/core] fork: Redo ifdefs around task stack handling

The following commit has been merged into the core/core branch of tip:

Commit-ID: be9a2277cafd318976d59c41a7f45a934ec43b26
Gitweb: https://git.kernel.org/tip/be9a2277cafd318976d59c41a7f45a934ec43b26
Author: Sebastian Andrzej Siewior <[email protected]>
AuthorDate: Thu, 17 Feb 2022 11:23:59 +01:00
Committer: Thomas Gleixner <[email protected]>
CommitterDate: Tue, 22 Feb 2022 22:25:01 +01:00

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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

---
kernel/fork.c | 74 ++++++++++++++++++++++++++------------------------
1 file changed, 39 insertions(+), 35 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index a024bf6..f5cc101 100644
--- 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