Re: [PATCH net-next 4/6] net: ipa: ensure hardware has power in ipa_start_xmit()

On 8/13/21 7:46 PM, Jakub Kicinski wrote:

On Thu, 12 Aug 2021 14:50:33 -0500 Alex Elder wrote:

quoted

+	/* The hardware must be powered for us to transmit */
+	dev = &ipa->pdev->dev;
+	ret = pm_runtime_get(dev);
+	if (ret < 1) {
+		/* If a resume won't happen, just drop the packet */
+		if (ret < 0 && ret != -EINPROGRESS) {
+			pm_runtime_put_noidle(dev);
+			goto err_drop_skb;
+		}

This is racy, what if the pm work gets scheduled on another CPU and
calls wake right here (i.e. before you call netif_stop_queue())?
The queue may never get woken up?

I haven't been seeing this happen but I think you may be right.

I did think about this race, but I think I was relying on the
PM work queue to somehow avoid the problem.  I need to think
about this again after a good night's sleep.  I might need
to add an atomic flag or something.

					-Alex

quoted

+		/* No power (yet).  Stop the network stack from transmitting
+		 * until we're resumed; ipa_modem_resume() arranges for the
+		 * TX queue to be started again.
+		 */
+		netif_stop_queue(netdev);
+
+		(void)pm_runtime_put(dev);
+
+		return NETDEV_TX_BUSY;