diff --git a/drivers/mailbox/mailbox-test.c b/drivers/mailbox/mailbox-test.c
index 4555d678fadd..6dd5b9614452 100644
--- a/drivers/mailbox/mailbox-test.c
+++ b/drivers/mailbox/mailbox-test.c
@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/mailbox_client.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/poll.h>
@@ -38,6 +39,7 @@ struct mbox_test_device {
 	char			*signal;
 	char			*message;
 	spinlock_t		lock;
+	struct mutex		mutex;
 	wait_queue_head_t	waitq;
 	struct fasync_struct	*async_queue;
 	struct dentry		*root_debugfs_dir;
@@ -110,6 +112,8 @@ static ssize_t mbox_test_message_write(struct file *filp,
 		return -EINVAL;
 	}
 
+	mutex_lock(&tdev->mutex);
+
 	tdev->message = kzalloc(MBOX_MAX_MSG_LEN, GFP_KERNEL);
 	if (!tdev->message)
 		return -ENOMEM;
@@ -144,6 +148,8 @@ out:
 	kfree(tdev->message);
 	tdev->signal = NULL;
 
+	mutex_unlock(&tdev->mutex);
+
 	return ret < 0 ? ret : count;
 }
 
@@ -392,6 +398,7 @@ static int mbox_test_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, tdev);
 
 	spin_lock_init(&tdev->lock);
+	mutex_init(&tdev->mutex);
 
 	if (tdev->rx_channel) {
 		tdev->rx_buffer = devm_kzalloc(&pdev->dev,