tracing/function-return-tracer: store return stack into task_struct and allocate it dynamically

Impact: use deeper function tracing depth safely

Some tests showed that function return tracing needed a more deeper depth
of function calls. But it could be unsafe to store these return addresses
to the stack.

So these arrays will now be allocated dynamically into task_struct of current
only when the tracer is activated.

Typical scheme when tracer is activated:
- allocate a return stack for each task in global list.
- fork: allocate the return stack for the newly created task
- exit: free return stack of current
- idle init: same as fork

I chose a default depth of 50. I don't have overruns anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 95 insertions, 1 deletions

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f212da486689..90d99fb02ae4 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1498,10 +1498,77 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
 
 #ifdef CONFIG_FUNCTION_RET_TRACER
 
+static atomic_t ftrace_retfunc_active;
+
 /* The callback that hooks the return of a function */
 trace_function_return_t ftrace_function_return =
                         (trace_function_return_t)ftrace_stub;
 
+
+/* Try to assign a return stack array on FTRACE_RETSTACK_ALLOC_SIZE tasks. */
+static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
+{
+        int i;
+        int ret = 0;
+        unsigned long flags;
+        int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
+        struct task_struct *g, *t;
+
+        for (i = 0; i < FTRACE_RETSTACK_ALLOC_SIZE; i++) {
+                ret_stack_list[i] = kmalloc(FTRACE_RETFUNC_DEPTH
+                                        * sizeof(struct ftrace_ret_stack),
+                                        GFP_KERNEL);
+                if (!ret_stack_list[i]) {
+                        start = 0;
+                        end = i;
+                        ret = -ENOMEM;
+                        goto free;
+                }
+        }
+
+        read_lock_irqsave(&tasklist_lock, flags);
+        do_each_thread(g, t) {
+                if (start == end) {
+                        ret = -EAGAIN;
+                        goto unlock;
+                }
+
+                if (t->ret_stack == NULL) {
+                        t->ret_stack = ret_stack_list[start++];
+                        t->curr_ret_stack = -1;
+                        atomic_set(&t->trace_overrun, 0);
+                }
+        } while_each_thread(g, t);
+
+unlock:
+        read_unlock_irqrestore(&tasklist_lock, flags);
+free:
+        for (i = start; i < end; i++)
+                kfree(ret_stack_list[i]);
+        return ret;
+}
+
+/* Allocate a return stack for each task */
+static int start_return_tracing(void)
+{
+        struct ftrace_ret_stack **ret_stack_list;
+        int ret;
+
+        ret_stack_list = kmalloc(FTRACE_RETSTACK_ALLOC_SIZE *
+                                sizeof(struct ftrace_ret_stack *),
+                                GFP_KERNEL);
+
+        if (!ret_stack_list)
+                return -ENOMEM;
+
+        do {
+                ret = alloc_retstack_tasklist(ret_stack_list);
+        } while (ret == -EAGAIN);
+
+        kfree(ret_stack_list);
+        return ret;
+}
+
 int register_ftrace_return(trace_function_return_t func)
 {
         int ret = 0;
@@ -1516,7 +1583,12 @@ int register_ftrace_return(trace_function_return_t func)
                 ret = -EBUSY;
                 goto out;
         }
-
+        atomic_inc(&ftrace_retfunc_active);
+        ret = start_return_tracing();
+        if (ret) {
+                atomic_dec(&ftrace_retfunc_active);
+                goto out;
+        }
         ftrace_tracing_type = FTRACE_TYPE_RETURN;
         ftrace_function_return = func;
         ftrace_startup();
@@ -1530,6 +1602,7 @@ void unregister_ftrace_return(void)
 {
         mutex_lock(&ftrace_sysctl_lock);
 
+        atomic_dec(&ftrace_retfunc_active);
         ftrace_function_return = (trace_function_return_t)ftrace_stub;
         ftrace_shutdown();
         /* Restore normal tracing type */
@@ -1537,6 +1610,27 @@ void unregister_ftrace_return(void)
 
         mutex_unlock(&ftrace_sysctl_lock);
 }
+
+/* Allocate a return stack for newly created task */
+void ftrace_retfunc_init_task(struct task_struct *t)
+{
+        if (atomic_read(&ftrace_retfunc_active)) {
+                t->ret_stack = kmalloc(FTRACE_RETFUNC_DEPTH
+                                * sizeof(struct ftrace_ret_stack),
+                                GFP_KERNEL);
+                if (!t->ret_stack)
+                        return;
+                t->curr_ret_stack = -1;
+                atomic_set(&t->trace_overrun, 0);
+        } else
+                t->ret_stack = NULL;
+}
+
+void ftrace_retfunc_exit_task(struct task_struct *t)
+{
+        kfree(t->ret_stack);
+        t->ret_stack = NULL;
+}
 #endif