1 files changed, 5 insertions, 4 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index cd3b2438a980..9b4d51d0c0d0 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -165,10 +165,11 @@ void set_task_blockstep(struct task_struct *task, bool on)
          * 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().
+         *
+         * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
+         * task is current or it can't be running, otherwise we can race
+         * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
+         * PTRACE_KILL is not safe.
          */
         local_irq_disable();
         debugctl = get_debugctlmsr();