diff --git a/include/input.h b/include/input.h
index bb58b2200..6c9e45df6 100644
--- a/include/input.h
+++ b/include/input.h
@@ -722,6 +722,7 @@ extern _X_HIDDEN void input_constrain_cursor(DeviceIntPtr pDev, ScreenPtr screen
 extern _X_EXPORT void input_lock(void);
 extern _X_EXPORT void input_unlock(void);
 extern _X_EXPORT void input_force_unlock(void);
+extern _X_EXPORT int in_input_thread(void);
 
 extern void InputThreadPreInit(void);
 extern void InputThreadInit(void);
diff --git a/os/inputthread.c b/os/inputthread.c
index 8e7f2edb9..721e86312 100644
--- a/os/inputthread.c
+++ b/os/inputthread.c
@@ -90,6 +90,13 @@ static pthread_mutex_t input_mutex;
 static Bool input_mutex_initialized;
 #endif
 
+int
+in_input_thread(void)
+{
+    return inputThreadInfo &&
+           pthread_equal(pthread_self(), inputThreadInfo->thread);
+}
+
 void
 input_lock(void)
 {
@@ -529,6 +536,7 @@ void input_force_unlock(void) {}
 void InputThreadPreInit(void) {}
 void InputThreadInit(void) {}
 void InputThreadFini(void) {}
+int in_input_thread(void) { return 0; }
 
 int InputThreadRegisterDev(int fd,
                            NotifyFdProcPtr readInputProc,
diff --git a/os/io.c b/os/io.c
index 5ba1e86b1..234c0f33f 100644
--- a/os/io.c
+++ b/os/io.c
@@ -652,6 +652,9 @@ WriteToClient(ClientPtr who, int count, const void *__buf)
     int padBytes;
     const char *buf = __buf;
 
+    BUG_RETURN_VAL_MSG(in_input_thread(), 0,
+                       "******** %s called from input thread *********\n", __func__);
+
 #ifdef DEBUG_COMMUNICATION
     Bool multicount = FALSE;
 #endif