powerpc: prep stack walkers for THREAD_INFO_IN_TASK

[text copied from commit 9bbd4c56
("arm64: prep stack walkers for THREAD_INFO_IN_TASK")]

When CONFIG_THREAD_INFO_IN_TASK is selected, task stacks may be freed
before a task is destroyed. To account for this, the stacks are
refcounted, and when manipulating the stack of another task, it is
necessary to get/put the stack to ensure it isn't freed and/or re-used
while we do so.

This patch reworks the powerpc stack walking code to account for this.
When CONFIG_THREAD_INFO_IN_TASK is not selected these perform no
refcounting, and this should only be a structural change that does not
affect behaviour.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Move try_get_task_stack() below tsk == NULL check in show_stack()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -2027,7 +2027,7 @@ int validate_sp(unsigned long sp, struct task_struct *p,
 
 EXPORT_SYMBOL(validate_sp);
 
-unsigned long get_wchan(struct task_struct *p)
+static unsigned long __get_wchan(struct task_struct *p)
 {
 	unsigned long ip, sp;
 	int count = 0;
@@ -2053,6 +2053,20 @@ unsigned long get_wchan(struct task_struct *p)
 	return 0;
 }
 
+unsigned long get_wchan(struct task_struct *p)
+{
+	unsigned long ret;
+
+	if (!try_get_task_stack(p))
+		return 0;
+
+	ret = __get_wchan(p);
+
+	put_task_stack(p);
+
+	return ret;
+}
+
 static int kstack_depth_to_print = CONFIG_PRINT_STACK_DEPTH;
 
 void show_stack(struct task_struct *tsk, unsigned long *stack)
@@ -2067,9 +2081,13 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
 	int curr_frame = 0;
 #endif
 
-	sp = (unsigned long) stack;
 	if (tsk == NULL)
 		tsk = current;
+
+	if (!try_get_task_stack(tsk))
+		return;
+
+	sp = (unsigned long) stack;
 	if (sp == 0) {
 		if (tsk == current)
 			sp = current_stack_pointer();
@@ -2081,7 +2099,7 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
 	printk("Call Trace:\n");
 	do {
 		if (!validate_sp(sp, tsk, STACK_FRAME_OVERHEAD))
-			return;
+			break;
 
 		stack = (unsigned long *) sp;
 		newsp = stack[0];
@@ -2121,6 +2139,8 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
 
 		sp = newsp;
 	} while (count++ < kstack_depth_to_print);
+
+	put_task_stack(tsk);
 }
 
 #ifdef CONFIG_PPC64

arch/powerpc/kernel/stacktrace.c

@@ -67,12 +67,17 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 {
 	unsigned long sp;
 
+	if (!try_get_task_stack(tsk))
+		return;
+
 	if (tsk == current)
 		sp = current_stack_pointer();
 	else
 		sp = tsk->thread.ksp;
 
 	save_context_stack(trace, sp, tsk, 0);
+
+	put_task_stack(tsk);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
 
@@ -90,9 +95,8 @@ EXPORT_SYMBOL_GPL(save_stack_trace_regs);
  *
  * If the task is not 'current', the caller *must* ensure the task is inactive.
  */
-int
-save_stack_trace_tsk_reliable(struct task_struct *tsk,
-				struct stack_trace *trace)
+static int __save_stack_trace_tsk_reliable(struct task_struct *tsk,
+					   struct stack_trace *trace)
 {
 	unsigned long sp;
 	unsigned long newsp;
@@ -197,6 +201,25 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
 	}
 	return 0;
 }
+
+int save_stack_trace_tsk_reliable(struct task_struct *tsk,
+				  struct stack_trace *trace)
+{
+	int ret;
+
+	/*
+	 * If the task doesn't have a stack (e.g., a zombie), the stack is
+	 * "reliably" empty.
+	 */
+	if (!try_get_task_stack(tsk))
+		return 0;
+
+	ret = __save_stack_trace_tsk_reliable(tsk, trace);
+
+	put_task_stack(tsk);
+
+	return ret;
+}
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk_reliable);
 #endif /* CONFIG_HAVE_RELIABLE_STACKTRACE */