linux-kernel - Re: [PATCH net-next 4/6] net: ipa: ensure hardware has power in ipa_start

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Date:   Fri, 13 Aug 2021 21:25:23 -0500
From:   Alex Elder <elder@...aro.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, bjorn.andersson@...aro.org,
        evgreen@...omium.org, cpratapa@...eaurora.org,
        subashab@...eaurora.org, elder@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: 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:
>> +	/* 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

>> +		/* 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;