diff --git a/drivers/staging/comedi/drivers/comedi_test.c b/drivers/staging/comedi/drivers/comedi_test.c
index b8fd4d24b421..8c348bbf9e71 100644
--- a/drivers/staging/comedi/drivers/comedi_test.c
+++ b/drivers/staging/comedi/drivers/comedi_test.c
@@ -56,6 +56,10 @@ zero volts).
 
 #define N_CHANS 8
 
+enum waveform_state_bits {
+	WAVEFORM_AI_RUNNING = 0
+};
+
 /* Data unique to this driver */
 struct waveform_private {
 	struct timer_list timer;
@@ -65,6 +69,7 @@ struct waveform_private {
 	unsigned long usec_current;	/* current time (mod waveform period) */
 	unsigned long usec_remainder;	/* usec since last scan */
 	unsigned long ai_count;		/* number of conversions remaining */
+	unsigned long state_bits;
 	unsigned int scan_period;	/* scan period in usec */
 	unsigned int convert_period;	/* conversion period in usec */
 	unsigned int ao_loopbacks[N_CHANS];
@@ -175,6 +180,10 @@ static void waveform_ai_interrupt(unsigned long arg)
 	ktime_t now;
 	bool stopping = false;
 
+	/* check command is still active */
+	if (!test_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits))
+		return;
+
 	now = ktime_get();
 
 	elapsed_time = ktime_to_us(ktime_sub(now, devpriv->last));
@@ -322,6 +331,10 @@ static int waveform_ai_cmd(struct comedi_device *dev,
 	devpriv->usec_remainder = 0;
 
 	devpriv->timer.expires = jiffies + 1;
+	/* mark command as active */
+	smp_mb__before_atomic();
+	set_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits);
+	smp_mb__after_atomic();
 	add_timer(&devpriv->timer);
 	return 0;
 }
@@ -331,7 +344,11 @@ static int waveform_ai_cancel(struct comedi_device *dev,
 {
 	struct waveform_private *devpriv = dev->private;
 
-	del_timer_sync(&devpriv->timer);
+	/* mark command as no longer active */
+	clear_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits);
+	smp_mb__after_atomic();
+	/* cannot call del_timer_sync() as may be called from timer routine */
+	del_timer(&devpriv->timer);
 	return 0;
 }
 
@@ -433,7 +450,7 @@ static void waveform_detach(struct comedi_device *dev)
 	struct waveform_private *devpriv = dev->private;
 
 	if (devpriv)
-		waveform_ai_cancel(dev, dev->read_subdev);
+		del_timer_sync(&devpriv->timer);
 }
 
 static struct comedi_driver waveform_driver = {