diff --git a/[refs] b/[refs]
index 6a1e53d1719a..397188fd5702 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 848e8f5f0ad3169560c516fff6471be65f76e69f
+refs/heads/master: 95cf00fa5d5e2a200a2c044c84bde8389a237e02
diff --git a/trunk/arch/x86/kernel/step.c b/trunk/arch/x86/kernel/step.c
index 7a514986ca09..f89cdc6ccd5b 100644
--- a/trunk/arch/x86/kernel/step.c
+++ b/trunk/arch/x86/kernel/step.c
@@ -161,6 +161,16 @@ static void set_task_blockstep(struct task_struct *task, bool on)
 {
 	unsigned long debugctl;
 
+	/*
+	 * Ensure irq/preemption can't change debugctl in between.
+	 * Note also that both TIF_BLOCKSTEP and debugctl should
+	 * be changed atomically wrt preemption.
+	 * FIXME: this means that set/clear TIF_BLOCKSTEP is simply
+	 * wrong if task != current, SIGKILL can wakeup the stopped
+	 * tracee and set/clear can play with the running task, this
+	 * can confuse the next __switch_to_xtra().
+	 */
+	local_irq_disable();
 	debugctl = get_debugctlmsr();
 	if (on) {
 		debugctl |= DEBUGCTLMSR_BTF;
@@ -169,7 +179,9 @@ static void set_task_blockstep(struct task_struct *task, bool on)
 		debugctl &= ~DEBUGCTLMSR_BTF;
 		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 	}
-	update_debugctlmsr(debugctl);
+	if (task == current)
+		update_debugctlmsr(debugctl);
+	local_irq_enable();
 }
 
 /*