netdev - Re: [PATCH V5 net-next 2/3] net: huawei_cdc_ncm: Introduce the huawei_cdc

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Date:	Fri, 01 Nov 2013 12:35:45 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Enrico Mioso <mrkiko.rs@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Steve Glendinning <steve.glendinning@...well.net>,
	Robert de Vries <rhdv@...all.nl>,
	Hayes Wang <hayeswang@...ltek.com>,
	Freddy Xin <freddy@...x.com.tw>,
	Liu Junliang <liujunliang_ljl@....com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list\:USB NETWORKING DR..." <linux-usb@...r.kernel.org>,
	"open list\:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
	ModemManager-devel@...ts.freedesktop.org
Subject: Re: [PATCH V5 net-next 2/3] net: huawei_cdc_ncm: Introduce the huawei_cdc_ncm driver

Oliver Neukum <oneukum@...e.de> writes:
> On Mon, 2013-09-30 at 04:50 +0000, Enrico Mioso wrote:
>
>> +static int huawei_cdc_ncm_manage_power(struct usbnet *usbnet_dev, int on)
>> +{
>> +	struct huawei_cdc_ncm_state *drvstate = (void *)&usbnet_dev->data;
>> +	int rv = 0;
>> +
>> +	if ((on && atomic_add_return(1, &drvstate->pmcount) == 1) ||
>> +			(!on && atomic_dec_and_test(&drvstate->pmcount))) {
>> +		rv = usb_autopm_get_interface(usbnet_dev->intf);
>> +		if (rv < 0)
>> +			goto err;
>
> The error case corrupts drvstate->pmcount
>
>> +		usbnet_dev->intf->needs_remote_wakeup = on;
>> +		usb_autopm_put_interface(usbnet_dev->intf);
>> +	}
>> +err:
>> +	return rv;
>> +}

Hello Oliver,

I finally got around to looking closer at this and you are of course
correct.  This is all my fault when I initially wrapped the
needs_remote_wakeup update in usb_autopm_{get,put}_interface,

And the problem is not only here in this new driver, but *everywhere* I
did this, including in cdc-wdm.  That driver doesn't have the counter to
corrupt, but failing to update intf->needs_remote_wakeup if
usb_autopm_get_interface fails is still wrong. The get/put were only
added to make the change take effect immediately.  Things sort of worked
fine before that, and ignoring a get error would only revert to that
older behaviour.

So I believe we should do the update unconditionally, and but skip
usb_autopm_put_interface if the get failed.  Accordingly, these
functions should always return 0 (not that there is anything currently
checking the return anyway).

I'll prepare patches for cdc-wdm, qmi_wwan and cdc_mbim.

Bjørn
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