diff --git a/coverage/ctracer/tracer.c b/coverage/ctracer/tracer.c
index c05dff643..18dcb815b 100644
--- a/coverage/ctracer/tracer.c
+++ b/coverage/ctracer/tracer.c
@@ -982,8 +982,8 @@ static PyObject *
 CTracer_start(CTracer *self, PyObject *args_unused)
 {
     PyEval_SetTrace((Py_tracefunc)CTracer_trace, (PyObject*)self);
-    self->started = TRUE;
     self->tracing_arcs = self->trace_arcs && PyObject_IsTrue(self->trace_arcs);
+    atomic_store(&self->started, TRUE);
 
     /* start() returns a trace function usable with sys.settrace() */
     Py_INCREF(self);
@@ -993,13 +993,11 @@ CTracer_start(CTracer *self, PyObject *args_unused)
 static PyObject *
 CTracer_stop(CTracer *self, PyObject *args_unused)
 {
-    if (self->started) {
-        /* Set the started flag only. The actual call to
-           PyEval_SetTrace(NULL, NULL) is delegated to the callback
-           itself to ensure that it called from the right thread.
-           */
-        self->started = FALSE;
-    }
+    /* Set the started flag only. The actual call to
+       PyEval_SetTrace(NULL, NULL) is delegated to the callback
+       itself to ensure that it called from the right thread.
+       */
+    atomic_store(&self->started, FALSE);
 
     Py_RETURN_NONE;
 }
diff --git a/coverage/ctracer/tracer.h b/coverage/ctracer/tracer.h
index b00134de9..9db97a677 100644
--- a/coverage/ctracer/tracer.h
+++ b/coverage/ctracer/tracer.h
@@ -32,7 +32,7 @@ typedef struct CTracer {
     PyObject * disable_plugin;
 
     /* Has the tracer been started? */
-    BOOL started;
+    _Atomic BOOL started;
     /* Are we tracing arcs, or just lines? */
     BOOL tracing_arcs;
     /* Have we had any activity? */
diff --git a/coverage/ctracer/util.h b/coverage/ctracer/util.h
index 94591e0d2..cef8e0c4d 100644
--- a/coverage/ctracer/util.h
+++ b/coverage/ctracer/util.h
@@ -5,6 +5,7 @@
 #define _COVERAGE_UTIL_H
 
 #include <Python.h>
+#include <stdatomic.h>
 
 /* Compile-time debugging helpers */
 #undef WHAT_LOG         /* Define to log the WHAT params in the trace function. */
diff --git a/setup.py b/setup.py
index 92fad6502..45c501adf 100644
--- a/setup.py
+++ b/setup.py
@@ -155,6 +155,11 @@ def run(self):
 
     def build_extension(self, ext):
         """Wrap `build_extension` with `BuildFailed`."""
+        if self.compiler.compiler_type == "msvc":
+            ext.extra_compile_args = (ext.extra_compile_args or []) + [
+                "/std:c11",
+                "/experimental:c11atomics",
+            ]
         try:
             # Uncomment to test compile failure handling:
             #   raise errors.CCompilerError("OOPS")