Date:   Thu, 1 Dec 2016 14:35:27 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     mathias.nyman@...el.com, Greg KH <gregkh@...uxfoundation.org>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC] usb: host: xhci: Remove the watchdog timer and use command
 timer to watch stop endpoint command

Hi,

On 12/01/2016 02:04 PM, Baolin Wang wrote:
> Hi Baolu,
>
> On 1 December 2016 at 13:45, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>> Hi,
>>
>> On 11/30/2016 05:02 PM, Baolin Wang wrote:
>>> If the hardware never responds to the stop endpoint command, the
>>> URBs will never be completed, and we might hang the USB subsystem.
>>> The original watchdog timer is used to watch if one stop endpoint
>>> command is timeout, if timeout, then the watchdog timer will set
>>> XHCI_STATE_DYING, try to halt the xHCI host, and give back all
>>> pending URBs.
>>>
>>> But now we already have one command timer to control command timeout,
>>> thus we can also use the command timer to watch the stop endpoint
>>> command, instead of one duplicate watchdog timer which need to be
>>> removed.
>>>
>>> Meanwhile we don't need the 'stop_cmds_pending' flag to identy if
>>> this is the last stop endpoint command of one endpoint. Since we
>>> can make sure we only set one stop endpoint command for one endpoint
>>> by 'EP_HALT_PENDING' flag in xhci_urb_dequeue() function. Thus remove
>>> this flag.
>> I am afraid you can't do this. "stop_cmds_pending" was added
>> to fix the problem described in the comments that you want to
>> remove. But I didn't find any fix of this problem in your patch.
> Now we can not pending another stop endpoint command for the same one
> endpoint, since will check 'EP_HALT_PENDING' flag in
> xhci_urb_dequeue() function to avoid this. But after some
> investigation, I think I missed the stop endpoint command in
> xhci_stop_device() which did not check the 'EP_HALT_PENDING' flag,
> maybe need to add 'EP_HALT_PENDING' flag checking in
> xhci_stop_device() function. DId I miss something else? Thanks.

Consider below three threads running on different CPUs at the same time.

Thread A: xhci_handle_cmd_stop_ep()  --- in interrupt handler
Thread B: xhci_stop_endpoint_command_timeout() --- timer expired
Thread C: xhci_urb_dequeue --- called by usb core

They are serialized by xhci->lock. Let's consider below sequence:

Thread A:
    - delete xhci->cmd_timer), but will fail due to Thread B.
    - clear EP_HALT_PENDING bit

Thread B:
    - halt the host controller

Thread C:
    - set EP_HALT_PENDING bit
    - enqueue another stop endpoint command
    - add the timer back

Best regards,
Lu Baolu

>
>> - * The timer may also fire if the host takes a very long time to respond to the
>> - * command, and the stop endpoint command completion handler cannot delete the
>> - * timer before the timer function is called.  Another endpoint cancellation may
>> - * sneak in before the timer function can grab the lock, and that may queue
>> - * another stop endpoint command and add the timer back.  So we cannot use a
>> - * simple flag to say whether there is a pending stop endpoint command for a
>> - * particular endpoint.
>> - *
>> - * Instead we use a combination of that flag and a counter for the number of
>> - * pending stop endpoint commands.  If the timer is the tail end of the last
>> - * stop endpoint command, and the endpoint's command is still pending, we assume
>> - * the host is dying.
>>
>> Best regards,
>> Lu Baolu
>>
>>> We also need to clean up the command queue before trying to halt the
>>> xHCI host in xhci_stop_endpoint_command_timeout() function.
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
>
>

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