rpmsg: char: Fix race between the release of rpmsg_eptdev and cdev

struct rpmsg_eptdev contains a struct cdev. The current code frees
the rpmsg_eptdev struct in rpmsg_eptdev_destroy(), but the cdev is
a managed object, therefore its release is not predictable and the
rpmsg_eptdev could be freed before the cdev is entirely released.

The cdev_device_add/del() API was created to address this issue
(see commit '233ed09d ("chardev: add helper function to register
char devs with a struct device")'), use it instead of cdev add/del().

Fixes: c0cdc19f ("rpmsg: Driver for user space endpoint interface")
Suggested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220110104706.v6.2.Idde68b05b88d4a2e6e54766c653f3a6d9e419ce6@changeid

drivers/rpmsg/rpmsg_char.c

@@ -93,7 +93,7 @@ static int rpmsg_eptdev_destroy(struct device *dev, void *data)
 	/* wake up any blocked readers */
 	wake_up_interruptible(&eptdev->readq);
 
-	device_del(&eptdev->dev);
+	cdev_device_del(&eptdev->cdev, &eptdev->dev);
 	put_device(&eptdev->dev);
 
 	return 0;
@@ -336,7 +336,6 @@ static void rpmsg_eptdev_release_device(struct device *dev)
 
 	ida_simple_remove(&rpmsg_ept_ida, dev->id);
 	ida_simple_remove(&rpmsg_minor_ida, MINOR(eptdev->dev.devt));
-	cdev_del(&eptdev->cdev);
 	kfree(eptdev);
 }
 
@@ -381,19 +380,13 @@ static int rpmsg_eptdev_create(struct rpmsg_ctrldev *ctrldev,
 	dev->id = ret;
 	dev_set_name(dev, "rpmsg%d", ret);
 
-	ret = cdev_add(&eptdev->cdev, dev->devt, 1);
+	ret = cdev_device_add(&eptdev->cdev, &eptdev->dev);
 	if (ret)
 		goto free_ept_ida;
 
 	/* We can now rely on the release function for cleanup */
 	dev->release = rpmsg_eptdev_release_device;
 
-	ret = device_add(dev);
-	if (ret) {
-		dev_err(dev, "device_add failed: %d\n", ret);
-		put_device(dev);
-	}
-
 	return ret;
 
 free_ept_ida: