1 files changed, 33 insertions, 20 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index c346d116148..cd3b2438a98 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -157,6 +157,33 @@ static int enable_single_step(struct task_struct *child)
         return 1;
 }
 
+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;
+                set_tsk_thread_flag(task, TIF_BLOCKSTEP);
+        } else {
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
+        }
+        if (task == current)
+                update_debugctlmsr(debugctl);
+        local_irq_enable();
+}
+
 /*
  * Enable single or block step.
  */
@@ -169,19 +196,10 @@ static void enable_step(struct task_struct *child, bool block)
          * So no one should try to use debugger block stepping in a program
          * that uses user-mode single stepping itself.
          */
-        if (enable_single_step(child) && block) {
-                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);
-        }
+        if (enable_single_step(child) && block)
+                set_task_blockstep(child, true);
+        else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
+                set_task_blockstep(child, false);
 }
 
 void user_enable_single_step(struct task_struct *child)
@@ -199,13 +217,8 @@ void user_disable_single_step(struct task_struct *child)
         /*
          * Make sure block stepping (BTF) is disabled.
          */
-        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);
-        }
+        if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
+                set_task_blockstep(child, false);
 
         /* Always clear TIF_SINGLESTEP... */
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);