ftrace: add ftrace_enabled sysctl to disable mcount function

This patch adds back the sysctl ftrace_enabled. This time it is
defaulted to on, if DYNAMIC_FTRACE is configured. When ftrace_enabled
is disabled, the ftrace function is set to the stub return.

If DYNAMIC_FTRACE is also configured, on ftrace_enabled = 0,
the registered ftrace functions will all be set to jmps, but no more
new calls to ftrace recording (used to find the ftrace calling sites)
will be called.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

3 files changed, 124 insertions, 18 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 90dbc0ee2046..ccd8537dbdb7 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -5,6 +5,12 @@
 
 #include <linux/linkage.h>
 
+extern int ftrace_enabled;
+extern int
+ftrace_enable_sysctl(struct ctl_table *table, int write,
+                     struct file *filp, void __user *buffer, size_t *lenp,
+                     loff_t *ppos);
+
 typedef void (*ftrace_func_t)(unsigned long ip, unsigned long parent_ip);
 
 struct ftrace_ops {
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 29116652dca8..efaf7c5500e9 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -46,6 +46,7 @@
 #include <linux/nfs_fs.h>
 #include <linux/acpi.h>
 #include <linux/reboot.h>
+#include <linux/ftrace.h>
 
 #include <asm/uaccess.h>
 #include <asm/processor.h>
@@ -455,6 +456,16 @@ static struct ctl_table kern_table[] = {
                 .mode           = 0644,
                 .proc_handler   = &proc_dointvec,
         },
+#ifdef CONFIG_FTRACE
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "ftrace_enabled",
+                .data           = &ftrace_enabled,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &ftrace_enable_sysctl,
+        },
+#endif
 #ifdef CONFIG_KMOD
         {
                 .ctl_name       = KERN_MODPROBE,
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index d1ae2ba25274..d3de37299ba4 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -20,12 +20,24 @@
 #include <linux/hardirq.h>
 #include <linux/ftrace.h>
 #include <linux/module.h>
+#include <linux/sysctl.h>
 #include <linux/hash.h>
 #include <linux/list.h>
 
 #include "trace.h"
 
+#ifdef CONFIG_DYNAMIC_FTRACE
+# define FTRACE_ENABLED_INIT 1
+#else
+# define FTRACE_ENABLED_INIT 0
+#endif
+
+int ftrace_enabled = FTRACE_ENABLED_INIT;
+static int last_ftrace_enabled = FTRACE_ENABLED_INIT;
+
 static DEFINE_SPINLOCK(ftrace_lock);
+static DEFINE_MUTEX(ftrace_sysctl_lock);
+
 static struct ftrace_ops ftrace_list_end __read_mostly =
 {
         .func = ftrace_stub,
@@ -78,14 +90,16 @@ static int notrace __register_ftrace_function(struct ftrace_ops *ops)
         smp_wmb();
         ftrace_list = ops;
 
-        /*
-         * For one func, simply call it directly.
-         * For more than one func, call the chain.
-         */
-        if (ops->next == &ftrace_list_end)
-                ftrace_trace_function = ops->func;
-        else
-                ftrace_trace_function = ftrace_list_func;
+        if (ftrace_enabled) {
+                /*
+                 * For one func, simply call it directly.
+                 * For more than one func, call the chain.
+                 */
+                if (ops->next == &ftrace_list_end)
+                        ftrace_trace_function = ops->func;
+                else
+                        ftrace_trace_function = ftrace_list_func;
+        }
 
         spin_unlock(&ftrace_lock);
 
@@ -120,10 +134,12 @@ static int notrace __unregister_ftrace_function(struct ftrace_ops *ops)
 
         *p = (*p)->next;
 
-        /* If we only have one func left, then call that directly */
-        if (ftrace_list == &ftrace_list_end ||
-            ftrace_list->next == &ftrace_list_end)
-                ftrace_trace_function = ftrace_list->func;
+        if (ftrace_enabled) {
+                /* If we only have one func left, then call that directly */
+                if (ftrace_list == &ftrace_list_end ||
+                    ftrace_list->next == &ftrace_list_end)
+                        ftrace_trace_function = ftrace_list->func;
+        }
 
  out:
         spin_unlock(&ftrace_lock);
@@ -263,7 +279,8 @@ static void notrace ftrace_startup(void)
                 goto out;
         __unregister_ftrace_function(&ftrace_shutdown_ops);
 
-        ftrace_run_startup_code();
+        if (ftrace_enabled)
+                ftrace_run_startup_code();
  out:
         mutex_unlock(&ftraced_lock);
 }
@@ -275,13 +292,32 @@ static void notrace ftrace_shutdown(void)
         if (ftraced_suspend)
                 goto out;
 
-        ftrace_run_shutdown_code();
+        if (ftrace_enabled)
+                ftrace_run_shutdown_code();
 
         __register_ftrace_function(&ftrace_shutdown_ops);
  out:
         mutex_unlock(&ftraced_lock);
 }
 
+static void notrace ftrace_startup_sysctl(void)
+{
+        mutex_lock(&ftraced_lock);
+        /* ftraced_suspend is true if we want ftrace running */
+        if (ftraced_suspend)
+                ftrace_run_startup_code();
+        mutex_unlock(&ftraced_lock);
+}
+
+static void notrace ftrace_shutdown_sysctl(void)
+{
+        mutex_lock(&ftraced_lock);
+        /* ftraced_suspend is true if ftrace is running */
+        if (ftraced_suspend)
+                ftrace_run_shutdown_code();
+        mutex_unlock(&ftraced_lock);
+}
+
 static cycle_t          ftrace_update_time;
 static unsigned long    ftrace_update_cnt;
 unsigned long           ftrace_update_tot_cnt;
@@ -341,8 +377,9 @@ static int notrace ftraced(void *ignore)
                 /* check once a second */
                 schedule_timeout(HZ);
 
+                mutex_lock(&ftrace_sysctl_lock);
                 mutex_lock(&ftraced_lock);
-                if (ftraced_trigger && !ftraced_suspend) {
+                if (ftrace_enabled && ftraced_trigger && !ftraced_suspend) {
                         ftrace_record_suspend++;
                         ftrace_update_code();
                         usecs = nsecs_to_usecs(ftrace_update_time);
@@ -360,6 +397,7 @@ static int notrace ftraced(void *ignore)
                         ftrace_record_suspend--;
                 }
                 mutex_unlock(&ftraced_lock);
+                mutex_unlock(&ftrace_sysctl_lock);
 
                 ftrace_shutdown_replenish();
 
@@ -389,8 +427,10 @@ static