tracing/function-return-tracer: make the function return tracer lockless

Impact: remove spinlocks and irq disabling in function return tracer.

I've tried to figure out all of the race condition that could happen
when the tracer pushes or pops a return address trace to/from the
current thread_info.

Theory:

_ One thread can only execute on one cpu at a time. So this code
  doesn't need to be SMP-safe. Just drop the spinlock.

_ The only race could happen between the current thread and an
  interrupt. If an interrupt is raised, it will increase the index of
  the return stack storage and then execute until the end of the
  tracing to finally free the index it used. We don't need to disable
  irqs.

This is theorical. In practice, I've tested it with a two-core SMP and
had no problem at all. Perhaps -tip testing could confirm it.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/ftrace.c

@@ -44,62 +44,37 @@ void ftrace_nmi_exit(void)
 	atomic_dec(&in_nmi);
 }
 
-/*
- * Synchronize accesses to return adresses stack with
- * interrupts.
- */
-static raw_spinlock_t ret_stack_lock;
-
 /* Add a function return address to the trace stack on thread info.*/
 static int push_return_trace(unsigned long ret, unsigned long long time,
 				unsigned long func)
 {
 	int index;
-	struct thread_info *ti;
-	unsigned long flags;
-	int err = 0;
-
-	raw_local_irq_save(flags);
-	__raw_spin_lock(&ret_stack_lock);
+	struct thread_info *ti = current_thread_info();
 
-	ti = current_thread_info();
 	/* The return trace stack is full */
-	if (ti->curr_ret_stack == FTRACE_RET_STACK_SIZE - 1) {
-		err = -EBUSY;
-		goto out;
-	}
+	if (ti->curr_ret_stack == FTRACE_RET_STACK_SIZE - 1)
+		return -EBUSY;
 
 	index = ++ti->curr_ret_stack;
 	ti->ret_stack[index].ret = ret;
 	ti->ret_stack[index].func = func;
 	ti->ret_stack[index].calltime = time;
 
-out:
-	__raw_spin_unlock(&ret_stack_lock);
-	raw_local_irq_restore(flags);
-	return err;
+	return 0;
 }
 
 /* Retrieve a function return address to the trace stack on thread info.*/
 static void pop_return_trace(unsigned long *ret, unsigned long long *time,
 				unsigned long *func)
 {
-	struct thread_info *ti;
 	int index;
-	unsigned long flags;
-
-	raw_local_irq_save(flags);
-	__raw_spin_lock(&ret_stack_lock);
 
-	ti = current_thread_info();
+	struct thread_info *ti = current_thread_info();
 	index = ti->curr_ret_stack;
 	*ret = ti->ret_stack[index].ret;
 	*func = ti->ret_stack[index].func;
 	*time = ti->ret_stack[index].calltime;
 	ti->curr_ret_stack--;
-
-	__raw_spin_unlock(&ret_stack_lock);
-	raw_local_irq_restore(flags);
 }
 
 /*
@@ -175,14 +150,6 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
 		*parent = old;
 }
 
-static int __init init_ftrace_function_return(void)
-{
-	ret_stack_lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
-	return 0;
-}
-device_initcall(init_ftrace_function_return);
-
-
 #endif
 
 #ifdef CONFIG_DYNAMIC_FTRACE