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

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Message-ID: <6867439a-b995-86e7-6136-bcb8709eeb90@canonical.com>
Date:   Wed, 9 Mar 2022 18:56:31 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     Greg Kroah-Hartman <greg@...ah.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Felipe Balbi <balbi@...nel.org>, Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Cristian Birsan <cristian.birsan@...rochip.com>,
        Al Cooper <alcooperx@...il.com>, Li Yang <leoyang.li@....com>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Michal Simek <michal.simek@...inx.com>,
        "open list:USB GADGET/PERIPHERAL SUBSYSTEM" 
        <linux-usb@...r.kernel.org>, Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH v2 14/26] usb: gadget: s3c-hsudc: remove usage of list
 iterator past the loop body

On 09/03/2022 18:36, Jakob Koschel wrote:
> 
>> On 9. Mar 2022, at 18:25, Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com> wrote:
>>
>> 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@...il.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;
>> }
> 
> Ultimately I changed this based on the feedback from Linus
> (Link: https://lore.kernel.org/linux-kernel/CAHk-=wheru6rEfzC2wuO9k03PRF6s3nhxryCAnwR5bzKwPV2ww@mail.gmail.com/).

Not cleaner to me at all, but it's all personal opinion. :)

>>
>> ?
>> 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.
> 

Yeah, I understand. The code itself looks good, so:

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>


Best regards,
Krzysztof