Re: [PATCH v2 14/26] usb: gadget: s3c-hsudc: remove usage of list iterator past the loop body

From: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
To: Jakob Koschel <jakobkoschel@gmail.com>,
	Greg Kroah-Hartman <greg@kroah.com>
Cc: 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>
Subject: Re: [PATCH v2 14/26] usb: gadget: s3c-hsudc: remove usage of list iterator past the loop body
Date: Wed, 9 Mar 2022 18:25:48 +0100	[thread overview]
Message-ID: <a3fb72da-a9e3-04af-fa19-62765bf81c2b@canonical.com> (raw)
In-Reply-To: <20220308171818.384491-15-jakobkoschel@gmail.com>

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.
> 
> 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].
> 
> 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(-)
> 
> 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 = our_ep(_ep);
>  	struct s3c_hsudc *hsudc = hsep->dev;
> -	struct s3c_hsudc_req *hsreq;
> +	struct s3c_hsudc_req *hsreq = NULL, *iter;
>  	unsigned long flags;
>  
>  	hsep = our_ep(_ep);
> @@ -886,11 +886,13 @@ static int s3c_hsudc_dequeue(struct usb_ep *_ep, struct usb_request *_req)
>  
>  	spin_lock_irqsave(&hsudc->lock, flags);
>  
> -	list_for_each_entry(hsreq, &hsep->queue, queue) {
> -		if (&hsreq->req == _req)
> -			break;
> +	list_for_each_entry(iter, &hsep->queue, queue) {
> +		if (&iter->req != _req)
> +			continue;
> +		hsreq = iter;
> +		break;

You have in the loop both continue and break, looks a bit complicated.
Why not simpler:

if (&iter->req == _req) {
	hsreq = iter;
	break;
}

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

>  	}
> -	if (&hsreq->req != _req) {
> +	if (!hsreq) {
>  		spin_unlock_irqrestore(&hsudc->lock, flags);
>  		return -EINVAL;
>  	}

Best regards,
Krzysztof