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Subject: Re: [PATCH v2 14/26] usb: gadget: s3c-hsudc: remove usage of list
 iterator past the loop body
From:   Jakob Koschel <jakobkoschel@gmail.com>
In-Reply-To: <a3fb72da-a9e3-04af-fa19-62765bf81c2b@canonical.com>
Date:   Wed, 9 Mar 2022 18:36:14 +0100
Cc:     Greg Kroah-Hartman <greg@kroah.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Felipe Balbi <balbi@kernel.org>, Joel Stanley <joel@jms.id.au>,
        Andrew Jeffery <andrew@aj.id.au>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Claudiu Beznea <claudiu.beznea@microchip.com>,
        Cristian Birsan <cristian.birsan@microchip.com>,
        Al Cooper <alcooperx@gmail.com>, Li Yang <leoyang.li@nxp.com>,
        Vladimir Zapolskiy <vz@mleia.com>,
        Daniel Mack <daniel@zonque.org>,
        Haojian Zhuang <haojian.zhuang@gmail.com>,
        Robert Jarzmik <robert.jarzmik@free.fr>,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Michal Simek <michal.simek@xilinx.com>,
        "open list:USB GADGET/PERIPHERAL SUBSYSTEM" 
        <linux-usb@vger.kernel.org>, Mike Rapoport <rppt@kernel.org>,
        Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>
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Message-Id: <B269AAED-135D-4EAB-AA7C-26219581FB40@gmail.com>
References: <20220308171818.384491-1-jakobkoschel@gmail.com>
 <20220308171818.384491-15-jakobkoschel@gmail.com>
 <a3fb72da-a9e3-04af-fa19-62765bf81c2b@canonical.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Precedence: bulk


> On 9. Mar 2022, at 18:25, Krzysztof Kozlowski =
<krzysztof.kozlowski@canonical.com> wrote:
>=20
> On 08/03/2022 18:18, Jakob Koschel wrote:
>> If the list representing the request queue does not contain the =
expected
>> request, the value of the list_for_each_entry() iterator will not =
point
>> to a valid structure. To avoid type confusion in such case, the list
>> iterator scope will be limited to the list_for_each_entry() loop.
>>=20
>> In preparation to limiting scope of the list iterator to the list =
traversal
>> loop, use a dedicated pointer to point to the found request object =
[1].
>>=20
>> Link: https://lore.kernel.org/all/YhdfEIwI4EdtHdym@kroah.com/
>> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
>> ---
>> drivers/usb/gadget/udc/s3c-hsudc.c | 12 +++++++-----
>> 1 file changed, 7 insertions(+), 5 deletions(-)
>>=20
>> diff --git a/drivers/usb/gadget/udc/s3c-hsudc.c =
b/drivers/usb/gadget/udc/s3c-hsudc.c
>> index 89f1f8c9f02e..bf803e013458 100644
>> --- a/drivers/usb/gadget/udc/s3c-hsudc.c
>> +++ b/drivers/usb/gadget/udc/s3c-hsudc.c
>> @@ -877,7 +877,7 @@ static int s3c_hsudc_dequeue(struct usb_ep *_ep, =
struct usb_request *_req)
>> {
>> 	struct s3c_hsudc_ep *hsep =3D our_ep(_ep);
>> 	struct s3c_hsudc *hsudc =3D hsep->dev;
>> -	struct s3c_hsudc_req *hsreq;
>> +	struct s3c_hsudc_req *hsreq =3D NULL, *iter;
>> 	unsigned long flags;
>>=20
>> 	hsep =3D our_ep(_ep);
>> @@ -886,11 +886,13 @@ static int s3c_hsudc_dequeue(struct usb_ep =
*_ep, struct usb_request *_req)
>>=20
>> 	spin_lock_irqsave(&hsudc->lock, flags);
>>=20
>> -	list_for_each_entry(hsreq, &hsep->queue, queue) {
>> -		if (&hsreq->req =3D=3D _req)
>> -			break;
>> +	list_for_each_entry(iter, &hsep->queue, queue) {
>> +		if (&iter->req !=3D _req)
>> +			continue;
>> +		hsreq =3D iter;
>> +		break;
>=20
> You have in the loop both continue and break, looks a bit complicated.
> Why not simpler:
>=20
> if (&iter->req =3D=3D _req) {
> 	hsreq =3D iter;
> 	break;
> }

Ultimately I changed this based on the feedback from Linus
(Link: =
https://lore.kernel.org/linux-kernel/CAHk-=3Dwheru6rEfzC2wuO9k03PRF6s3nhxr=
yCAnwR5bzKwPV2ww@mail.gmail.com/).

>=20
> ?
> Less code, typical (expected) code flow when looking for something in
> the list. Is your approach related to some speculative execution?

no relation to speculative execution.

I have no preference for one over the other, so I'll just change it to
however it is desired.

It would just be great to have a (somewhat) consistent way so I can =
prepare
the rest of the patch sets accordingly.

>=20
>> 	}
>> -	if (&hsreq->req !=3D _req) {
>> +	if (!hsreq) {
>> 		spin_unlock_irqrestore(&hsudc->lock, flags);
>> 		return -EINVAL;
>> 	}
>=20
>=20
> Best regards,
> Krzysztof

Jakob=