linux-kernel - Re: [PATCH] usbip: vhci_hcd: slighly simplify code in 'vhci_urb

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Date:   Mon, 23 Mar 2020 17:56:46 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     shuah <shuah@...nel.org>, valentina.manea.m@...il.com,
        gregkh@...uxfoundation.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] usbip: vhci_hcd: slighly simplify code in
 'vhci_urb_dequeue()'

Le 23/03/2020 à 17:48, shuah a écrit :
> On 3/21/20 9:29 AM, Christophe JAILLET wrote:
>> The allocation of 'unlink' can be moved before a spin_lock.
>> This slighly simplifies the error handling if the memory allocation 
>> fails,
>
> slightly (spelling nit)
>
>> aligns the code structure with what is done in 'vhci_tx_urb()' and 
>> reduces
>> potential lock contention.
>>
>
> Are you seeing any problems or is this a potential lock contention?
> If you are seeing issues, please share the problem seen.
>
No, the issue is just theoretical.


>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>>   drivers/usb/usbip/vhci_hcd.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>> index 65850e9c7190..b909a634260c 100644
>> --- a/drivers/usb/usbip/vhci_hcd.c
>> +++ b/drivers/usb/usbip/vhci_hcd.c
>> @@ -905,17 +905,16 @@ static int vhci_urb_dequeue(struct usb_hcd 
>> *hcd, struct urb *urb, int status)
>>           /* tcp connection is alive */
>>           struct vhci_unlink *unlink;
>>   -        spin_lock(&vdev->priv_lock);
>> -
>
> This change might simplify the error path, however it could
> open a race window with the unlink activity during 
> vhci_shutdown_connection() when the connection is being taken
> down. It would be safer to hold both locks as soon as the
> connection check is done.

My proposal was just a small clean-up (from my point of view at least).
If it can have some side effects, please, just consider it as a NACK.

CJ

>
>>           /* setup CMD_UNLINK pdu */
>>           unlink = kzalloc(sizeof(struct vhci_unlink), GFP_ATOMIC);
>>           if (!unlink) {
>> -            spin_unlock(&vdev->priv_lock);
>>               spin_unlock_irqrestore(&vhci->lock, flags);
>>               usbip_event_add(&vdev->ud, VDEV_EVENT_ERROR_MALLOC);
>>               return -ENOMEM;
>>           }
>>   +        spin_lock(&vdev->priv_lock);
>> +
>>           unlink->seqnum = atomic_inc_return(&vhci_hcd->seqnum);
>>           if (unlink->seqnum == 0xffff)
>>               pr_info("seqnum max\n");
>>
>
> thanks,
> -- Shuah
>