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Date:   Wed, 11 Aug 2021 19:28:55 +0530
From:   Anirudh Rayabharam <mail@anirudhrb.com>
To:     Shuah Khan <skhan@linuxfoundation.org>
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, mail@anirudhrb.com
Subject: Re: [PATCH v2] usbip: give back URBs for unsent unlink requests
 during cleanup
Message-ID: <YRPXnzpyHETZ9bn2@anirudhrb.com>
References: <20210806181335.2078-1-mail@anirudhrb.com>
 <4aaf420d-e85e-212e-3bc4-a70e016de610@linuxfoundation.org>
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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?

Thanks for the review!

	- Anirudh.