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Subject: Re: [PATCH v2] usbip: give back URBs for unsent unlink requests
 during cleanup
To:     Anirudh Rayabharam <mail@anirudhrb.com>
Cc:     Valentina Manea <valentina.manea.m@gmail.com>,
        Shuah Khan <shuah@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-usb@vger.kernel.org, Shuah Khan <skhan@linuxfoundation.org>
References: <20210806181335.2078-1-mail@anirudhrb.com>
 <4aaf420d-e85e-212e-3bc4-a70e016de610@linuxfoundation.org>
 <YRPXnzpyHETZ9bn2@anirudhrb.com>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <4af0d8b6-c0c8-9e49-68ed-90bac5e16966@linuxfoundation.org>
Date:   Wed, 11 Aug 2021 15:51:29 -0600
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On 8/11/21 7:58 AM, Anirudh Rayabharam wrote:
> On Tue, Aug 10, 2021 at 05:25:51PM -0600, Shuah Khan wrote:
>> On 8/6/21 12:13 PM, Anirudh Rayabharam wrote:
>>> In vhci_device_unlink_cleanup(), the URBs for unsent unlink requests are
>>> not given back. This sometimes causes usb_kill_urb to wait indefinitely
>>> for that urb to be given back. syzbot has reported a hung task issue [1]
>>> for this.
>>>
>>> To fix this, give back the urbs corresponding to unsent unlink requests
>>> (unlink_tx list) similar to how urbs corresponding to unanswered unlink
>>> requests (unlink_rx list) are given back. Since the code is almost the
>>> same, extract it into a new function and call it for both unlink_rx and
>>> unlink_tx lists.
>>>
>>
>> Let's not do the refactor - let's first fix the problem and then the refactor.
> 
> Sure, I will make it a two patch series where the first one fixes the
> problem and the second one does the refactor.
> 
>>
>>> [1]: https://syzkaller.appspot.com/bug?id=08f12df95ae7da69814e64eb5515d5a85ed06b76
>>>
>>> Reported-by: syzbot+74d6ef051d3d2eacf428@syzkaller.appspotmail.com
>>> Tested-by: syzbot+74d6ef051d3d2eacf428@syzkaller.appspotmail.com
>>> Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
>>> ---
>>>
>>> Changes in v2:
>>> Use WARN_ON() instead of BUG() when unlink_list is neither unlink_tx nor
>>> unlink_rx.
>>>
>>> v1: https://lore.kernel.org/lkml/20210806164015.25263-1-mail@anirudhrb.com/
>>>
>>> ---
>>>    drivers/usb/usbip/vhci_hcd.c | 45 +++++++++++++++++++++++++-----------
>>>    1 file changed, 32 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>>> index 4ba6bcdaa8e9..67e638f4c455 100644
>>> --- a/drivers/usb/usbip/vhci_hcd.c
>>> +++ b/drivers/usb/usbip/vhci_hcd.c
>>> @@ -945,7 +945,8 @@ static int vhci_urb_dequeue(struct usb_hcd *hcd, struct urb *urb, int status)
>>>    	return 0;
>>>    }
>>> -static void vhci_device_unlink_cleanup(struct vhci_device *vdev)
>>> +static void __vhci_cleanup_unlink_list(struct vhci_device *vdev,
>>> +		struct list_head *unlink_list)
>>>    {
>>>    	struct vhci_hcd *vhci_hcd = vdev_to_vhci_hcd(vdev);
>>>    	struct usb_hcd *hcd = vhci_hcd_to_hcd(vhci_hcd);
>>> @@ -953,23 +954,23 @@ static void vhci_device_unlink_cleanup(struct vhci_device *vdev)
>>>    	struct vhci_unlink *unlink, *tmp;
>>>    	unsigned long flags;
>>> +	if (WARN(unlink_list != &vdev->unlink_tx
>>> +				&& unlink_list != &vdev->unlink_rx,
>>> +			"Invalid list passed to __vhci_cleanup_unlink_list\n"))
>>> +		return;
>>> +
>>
>> With this change, this will be only place unlink_rx is used without
>> vdev->priv_lock hold? Please explain why this is safe.
> 
> Well, this doesn't read or modify the contents of unlink_rx and unlink_tx.
> So, it looks safe to me. Let me know if I'm missing something here.
> 
>>
>>>    	spin_lock_irqsave(&vhci->lock, flags);
>>>    	spin_lock(&vdev->priv_lock);
>>> -	list_for_each_entry_safe(unlink, tmp, &vdev->unlink_tx, list) {
>>> -		pr_info("unlink cleanup tx %lu\n", unlink->unlink_seqnum);
>>> -		list_del(&unlink->list);
>>> -		kfree(unlink);
>>> -	}
>>> -
>>> -	while (!list_empty(&vdev->unlink_rx)) {
>>> +	list_for_each_entry_safe(unlink, tmp, unlink_list, list) {
>>>    		struct urb *urb;
>>> -		unlink = list_first_entry(&vdev->unlink_rx, struct vhci_unlink,
>>> -			list);
>>> -
>>> -		/* give back URB of unanswered unlink request */
>>> -		pr_info("unlink cleanup rx %lu\n", unlink->unlink_seqnum);
>>> +		if (unlink_list == &vdev->unlink_tx)
>>> +			pr_info("unlink cleanup tx %lu\n",
>>> +					unlink->unlink_seqnum);
>>> +		else
>>> +			pr_info("unlink cleanup rx %lu\n",
>>> +					unlink->unlink_seqnum);
>>>    		urb = pickup_urb_and_free_priv(vdev, unlink->unlink_seqnum);
>>>    		if (!urb) {
>>> @@ -1001,6 +1002,24 @@ static void vhci_device_unlink_cleanup(struct vhci_device *vdev)
>>>    	spin_unlock_irqrestore(&vhci->lock, flags);
>>>    }
>>> +static inline void vhci_cleanup_unlink_tx(struct vhci_device *vdev)
>>> +{
>>> +	__vhci_cleanup_unlink_list(vdev, &vdev->unlink_tx);
>>
>> With this change, this will be only place unlink_rx is used without
>> vdev->priv_lock hold? Please explain why this is safe.
>>
>>> +}
>>> +
>>
>> Is there a need for this layer?
>>
>>> +static inline void vhci_cleanup_unlink_rx(struct vhci_device *vdev)
>>> +{
>>> +	__vhci_cleanup_unlink_list(vdev, &vdev->unlink_rx);
>>
>> With this change, this will be only place unlink_rx is used without
>> vdev->priv_lock hold? Please explain why this is safe.
>>
>>> +}
>>> +
>> Is there a need for this layer?
> 
> I added these wrappers purely for convenience. There is no other purpose.
> Would you prefer this patch without the wrappers?
> 

Yes. Prefer it without the wrappers. When you take the wrappers
out, I think the unlink_rx could be within spinlock hold easily.

thanks,
-- Shuah