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Subject: Re: [PATCH net-next 4/6] net: ipa: ensure hardware has power in
 ipa_start_xmit()
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     davem@davemloft.net, bjorn.andersson@linaro.org,
        evgreen@chromium.org, cpratapa@codeaurora.org,
        subashab@codeaurora.org, elder@kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210812195035.2816276-1-elder@linaro.org>
 <20210812195035.2816276-5-elder@linaro.org>
 <20210813174655.1d13b524@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
From:   Alex Elder <elder@linaro.org>
Message-ID: <3a9e82cc-c09e-62e8-4671-8f16d4f6a35b@linaro.org>
Date:   Fri, 13 Aug 2021 21:25:23 -0500
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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;