1 files changed, 17 insertions, 29 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 3149032ff107..58de45ee08b6 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -158,22 +158,6 @@ static int enable_single_step(struct task_struct *child)
 }
 
 /*
- * Install this value in MSR_IA32_DEBUGCTLMSR whenever child is running.
- */
-static void write_debugctlmsr(struct task_struct *child, unsigned long val)
-{
-        if (child->thread.debugctlmsr == val)
-                return;
-
-        child->thread.debugctlmsr = val;
-
-        if (child != current)
-                return;
-
-        update_debugctlmsr(val);
-}
-
-/*
  * Enable single or block step.
  */
 static void enable_step(struct task_struct *child, bool block)
@@ -186,15 +170,17 @@ static void enable_step(struct task_struct *child, bool block)
          * that uses user-mode single stepping itself.
          */
         if (enable_single_step(child) && block) {
-                set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
-                write_debugctlmsr(child,
-                                  child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
-        } else {
-                write_debugctlmsr(child,
-                                  child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
-
-                if (!child->thread.debugctlmsr)
-                        clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl |= DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                set_tsk_thread_flag(child, TIF_BLOCKSTEP);
+        } else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                clear_tsk_thread_flag(child, TIF_BLOCKSTEP);
         }
 }
 
@@ -213,11 +199,13 @@ void user_disable_single_step(struct task_struct *child)
         /*
          * Make sure block stepping (BTF) is disabled.
          */
-        write_debugctlmsr(child,
-                          child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
+        if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
 
-        if (!child->thread.debugctlmsr)
-                clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                update_debugctlmsr(debugctl);
+                clear_tsk_thread_flag(child, TIF_BLOCKSTEP);
+        }
 
         /* Always clear TIF_SINGLESTEP... */
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);