diff --git a/os/hal/include/hal_queues.h b/os/hal/include/hal_queues.h
index 9bbe0924d..c8c664ddc 100644
--- a/os/hal/include/hal_queues.h
+++ b/os/hal/include/hal_queues.h
@@ -63,7 +63,7 @@ typedef void (*qnotify_t)(io_queue_t *qp);
  */
 struct io_queue {
   threads_queue_t       q_waiting;  /**< @brief Waiting thread.             */
-  size_t                q_counter;  /**< @brief Resources counter.          */
+  volatile size_t       q_counter;  /**< @brief Resources counter.          */
   uint8_t               *q_buffer;  /**< @brief Pointer to the queue buffer.*/
   uint8_t               *q_top;     /**< @brief Pointer to the first
                                          location after the buffer.         */
diff --git a/os/rt/include/chqueues.h b/os/rt/include/chqueues.h
index 2b4c660f3..e48429d14 100644
--- a/os/rt/include/chqueues.h
+++ b/os/rt/include/chqueues.h
@@ -76,7 +76,7 @@ typedef void (*qnotify_t)(io_queue_t *qp);
  */
 struct io_queue {
   threads_queue_t       q_waiting;  /**< @brief Queue of waiting threads.   */
-  size_t                q_counter;  /**< @brief Resources counter.          */
+  volatile size_t       q_counter;  /**< @brief Resources counter.          */
   uint8_t               *q_buffer;  /**< @brief Pointer to the queue buffer.*/
   uint8_t               *q_top;     /**< @brief Pointer to the first location
                                                 after the buffer.           */
diff --git a/readme.txt b/readme.txt
index 95b503475..28f375dbc 100644
--- a/readme.txt
+++ b/readme.txt
@@ -102,6 +102,8 @@
 - HAL: Introduced support for TIM21 and TIM22 in STM32 ST driver.
 - HAL: Updated STM32F0xx headers to STM32CubeF0 version 1.3.0. Added support
        for STM32F030xC, STM32F070x6, STM32F070xB devices.
+- HAL: Fixed volatile variable issue in I/O queues, both RT and HAL (bug #640)
+       (backported to 3.0.2).
 - HAL: Fixed wrong DMA assignment for I2C1 in STM32F302xC registry (bug #637)
        (backported to 3.0.2).
 - HAL: Fixed missing timers 5, 6, 7, 10 & 11 from STM32L1 HAL port (bug #636)