Re: [PATCH] wireless: rtl8187: replace udev with usb_get_dev()

From: Kalle Valo <kvalo@codeaurora.org>
To: Hin-Tak Leung <htl10@users.sourceforge.net>
Cc: Herton Ronaldo Krzesinski <herton@canonical.com>,
	Larry Finger <larry.finger@lwfinger.net>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	gregkh@linuxfoundation.org, Salah Triki <salah.triki@gmail.com>,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] wireless: rtl8187: replace udev with usb_get_dev()
Date: Tue, 27 Jul 2021 09:10:45 +0300	[thread overview]
Message-ID: <87im0wguca.fsf@codeaurora.org> (raw)
In-Reply-To: <53895498.1259278.1627160074135@mail.yahoo.com> (Hin-Tak Leung's message of "Sat, 24 Jul 2021 20:54:34 +0000 (UTC)")

Hin-Tak Leung <htl10@users.sourceforge.net> writes:

> On Saturday, 24 July 2021, 19:35:12 BST, Salah Triki <salah.triki@gmail.com> wrote:
>
>> Replace udev with usb_get_dev() in order to make code cleaner.
>
>> Signed-off-by: Salah Triki <salah.triki@gmail.com>
>> ---
>> drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c | 4 +---
>> 1 file changed, 1 insertion(+), 3 deletions(-)
>
>> diff --git a/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
> b/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
>> index eb68b2d3caa1..30bb3c2b8407 100644
>> --- a/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
>> +++ b/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
>> @@ -1455,9 +1455,7 @@ static int rtl8187_probe(struct usb_interface *intf,
>
>>     SET_IEEE80211_DEV(dev, &intf->dev);
>>     usb_set_intfdata(intf, dev);
>> -    priv->udev = udev;
>> -
>> -    usb_get_dev(udev);
>> +    priv->udev = usb_get_dev(udev);
>
>>     skb_queue_head_init(&priv->rx_queue);
>
>> -- 
>> 2.25.1
>
> It is not cleaner - the change is not functionally equivalent. Before
> the change, the reference count is increased after the assignment; and
> after the change, before the assignment. So my question is, does the
> reference count increasing a little earlier matters? What can go wrong
> between very short time where the reference count increases, and
> priv->udev not yet assigned? I think there might be a race condition
> where the probbe function is called very shortly twice. Especially if
> the time of running the reference count function is non-trivial.
>
> Larry, what do you think? 

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