1 files changed, 26 insertions, 0 deletions

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 309689245431..605ef196fdd6 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -30,14 +30,32 @@
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 
+/*
+ * modifying_code is set to notify NMIs that they need to use
+ * memory barriers when entering or exiting. But we don't want
+ * to burden NMIs with unnecessary memory barriers when code
+ * modification is not being done (which is most of the time).
+ *
+ * A mutex is already held when ftrace_arch_code_modify_prepare
+ * and post_process are called. No locks need to be taken here.
+ *
+ * Stop machine will make sure currently running NMIs are done
+ * and new NMIs will see the updated variable before we need
+ * to worry about NMIs doing memory barriers.
+ */
+static int modifying_code __read_mostly;
+static DEFINE_PER_CPU(int, save_modifying_code);
+
 int ftrace_arch_code_modify_prepare(void)
 {
         set_kernel_text_rw();
+        modifying_code = 1;
         return 0;
 }
 
 int ftrace_arch_code_modify_post_process(void)
 {
+        modifying_code = 0;
         set_kernel_text_ro();
         return 0;
 }
@@ -149,6 +167,11 @@ static void ftrace_mod_code(void)
 
 void ftrace_nmi_enter(void)
 {
+        __get_cpu_var(save_modifying_code) = modifying_code;
+
+        if (!__get_cpu_var(save_modifying_code))
+                return;
+
         if (atomic_inc_return(&nmi_running) & MOD_CODE_WRITE_FLAG) {
                 smp_rmb();
                 ftrace_mod_code();
@@ -160,6 +183,9 @@ void ftrace_nmi_enter(void)
 
 void ftrace_nmi_exit(void)
 {
+        if (!__get_cpu_var(save_modifying_code))
+                return;
+
         /* Finish all executions before clearing nmi_running */
         smp_mb();
         atomic_dec(&nmi_running);