[PATCH 3.16 066/132] cdc-acm: fix race between callback and unthrottle

[Ben Hutchings]

3.16.74-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Oliver Neukum <oneukum@xxxxxxx>

commit 36e59e0d70d6150e7a2155c54612ea875e88ce8d upstream.

Abn URB may be may marked free only after the buffer has been
processed or there is a small window during which it could
be submitted on another CPU and overwrite an unprocessed buffer

Signed-off-by: Oliver Neukum <oneukum@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/class/cdc-acm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -419,19 +419,21 @@ static void acm_read_bulk_callback(struc
 	struct acm_rb *rb = urb->context;
 	struct acm *acm = rb->instance;
 	unsigned long flags;
+	int status = urb->status;
 
 	dev_vdbg(&acm->data->dev, "%s - urb %d, len %d\n", __func__,
 					rb->index, urb->actual_length);
-	set_bit(rb->index, &acm->read_urbs_free);
 
 	if (!acm->dev) {
+		set_bit(rb->index, &acm->read_urbs_free);
 		dev_dbg(&acm->data->dev, "%s - disconnected\n", __func__);
 		return;
 	}
 
 	if (urb->status) {
+		set_bit(rb->index, &acm->read_urbs_free);
 		dev_dbg(&acm->data->dev, "%s - non-zero urb status: %d\n",
-							__func__, urb->status);
+							__func__, status);
 		if ((urb->status != -ENOENT) || (urb->actual_length == 0))
 			return;
 	}
@@ -439,6 +441,12 @@ static void acm_read_bulk_callback(struc
 	usb_mark_last_busy(acm->dev);
 
 	acm_process_read_urb(acm, urb);
+	/*
+	 * Unthrottle may run on another CPU which needs to see events
+	 * in the same order. Submission has an implict barrier
+	 */
+	smp_mb__before_atomic();
+	set_bit(rb->index, &acm->read_urbs_free);
 
 	/* throttle device if requested by tty */
 	spin_lock_irqsave(&acm->read_lock, flags);