1 files changed, 13 insertions, 1 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7a514986ca09..f89cdc6ccd5b 100644
--- a/arch/x86/kernel/step.c
+++ b/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();
 }
 
 /*