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Subject: Re: [PATCH v2] usbip: give back URBs for unsent unlink requests
 during cleanup
To:     Anirudh Rayabharam <mail@anirudhrb.com>,
        Valentina Manea <valentina.manea.m@gmail.com>,
        Shuah Khan <shuah@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-usb@vger.kernel.org,
        syzbot+74d6ef051d3d2eacf428@syzkaller.appspotmail.com,
        Shuah Khan <skhan@linuxfoundation.org>
References: <20210806181335.2078-1-mail@anirudhrb.com>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <4aaf420d-e85e-212e-3bc4-a70e016de610@linuxfoundation.org>
Date:   Tue, 10 Aug 2021 17:25:51 -0600
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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.

> [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.

>   	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?

> +static void vhci_device_unlink_cleanup(struct vhci_device *vdev)
> +{
> +	/* give back URBs of unsent unlink requests */
> +	vhci_cleanup_unlink_tx(vdev);
> +	/* give back URBs of unanswered unlink requests */
> +	vhci_cleanup_unlink_rx(vdev);
> +}
> +
>   /*
>    * The important thing is that only one context begins cleanup.
>    * This is why error handling and cleanup become simple.
> 

thanks,
-- Shuah