Re: [PATCH 2/2] usb: host: xhci: Handle the right timeout command

[Baolin Wang]

On 20 December 2016 at 14:39, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> Hi,
>
> On 12/20/2016 02:06 PM, Baolin Wang wrote:
>> Hi,
>>
>> On 20 December 2016 at 12:29, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
>>> Hi Mathias,
>>>
>>> On 12/19/2016 08:13 PM, Mathias Nyman wrote:
>>>> On 19.12.2016 13:34, Baolin Wang wrote:
>>>>> Hi Mathias,
>>>>>
>>>>> On 19 December 2016 at 18:33, Mathias Nyman
>>>>> <mathias.nyman@xxxxxxxxxxxxxxx> wrote:
>>>>>> On 13.12.2016 05:21, Baolin Wang wrote:
>>>>>>> Hi Mathias,
>>>>>>>
>>>>>>> On 12 December 2016 at 23:52, Mathias Nyman
>>>>>>> <mathias.nyman@xxxxxxxxxxxxxxx> wrote:
>>>>>>>> On 05.12.2016 09:51, Baolin Wang wrote:
>>>>>>>>>
>>>>>>>>> If a command event is found on the event ring during an interrupt,
>>>>>>>>> we need to stop the command timer with del_timer(). Since del_timer()
>>>>>>>>> can fail if the timer is running and waiting on the xHCI lock, then
>>>>>>>>> it maybe get the wrong timeout command in xhci_handle_command_timeout()
>>>>>>>>> if host fetched a new command and updated the xhci->current_cmd in
>>>>>>>>> handle_cmd_completion(). For this situation, we need a way to signal
>>>>>>>>> to the command timer that everything is fine and it should exit.
>>>>>>>>
>>>>>>>>
>>>>>>>> Ah, right, this could actually happen.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> We should introduce a counter (xhci->current_cmd_pending) for the number
>>>>>>>>> of pending commands. If we need to cancel the command timer and
>>>>>>>>> del_timer()
>>>>>>>>> succeeds, we decrement the number of pending commands. If del_timer()
>>>>>>>>> fails,
>>>>>>>>> we leave the number of pending commands alone.
>>>>>>>>>
>>>>>>>>> For handling timeout command, in xhci_handle_command_timeout() we will
>>>>>>>>> check
>>>>>>>>> the counter after decrementing it, if the counter
>>>>>>>>> (xhci->current_cmd_pending)
>>>>>>>>> is 0, which means xhci->current_cmd is the right timeout command. If the
>>>>>>>>> counter (xhci->current_cmd_pending) is greater than 0, which means
>>>>>>>>> current
>>>>>>>>> timeout command has been handled by host and host has fetched new
>>>>>>>>> command
>>>>>>>>> as
>>>>>>>>> xhci->current_cmd, then just return and wait for new current command.
>>>>>>>>
>>>>>>>>
>>>>>>>> A counter like this could work.
>>>>>>>>
>>>>>>>> Writing the abort bit can generate either ABORT+STOP, or just STOP
>>>>>>>> event, this seems to cover both.
>>>>>>>>
>>>>>>>> quick check, case 1: timeout and cmd completion at the same time.
>>>>>>>>
>>>>>>>> cpu1                                    cpu2
>>>>>>>>
>>>>>>>> queue_command(first), p++ (=1)
>>>>>>>> queue_command(more),
>>>>>>>> --completion irq fires--                -- timer times out at same time--
>>>>>>>> handle_cmd_completion()                 handle_cmd_timeout(),)
>>>>>>>> lock(xhci_lock  )                       spin_on(xhci_lock)
>>>>>>>> del_timer() fail, p (=1, nochange)
>>>>>>>> cur_cmd = list_next(), p++ (=2)
>>>>>>>> unlock(xhci_lock)
>>>>>>>>                                           lock(xhci_lock)
>>>>>>>>                                           p-- (=1)
>>>>>>>>                                           if (p > 0), exit
>>>>>>>> OK works
>>>>>>>>
>>>>>>>> case 2: normal timeout case with ABORT+STOP, no race.
>>>>>>>>
>>>>>>>> cpu1                                    cpu2
>>>>>>>>
>>>>>>>> queue_command(first), p++ (=1)
>>>>>>>> queue_command(more),
>>>>>>>>                                           handle_cmd_timeout()
>>>>>>>>                                           p-- (P=0), don't exit
>>>>>>>>                                           mod_timer(), p++ (P=1)
>>>>>>>>                                           write_abort_bit()
>>>>>>>> handle_cmd_comletion(ABORT)
>>>>>>>> del_timer(), ok, p-- (p = 0)
>>>>>>>> handle_cmd_completion(STOP)
>>>>>>>> del_timer(), fail, (P=0)
>>>>>>>> handle_stopped_cmd_ring()
>>>>>>>> cur_cmd = list_next(), p++ (=1)
>>>>>>>> mod_timer()
>>>>>>>>
>>>>>>>> OK, works, and same for just STOP case, with the only difference that
>>>>>>>> during handle_cmd_completion(STOP) p is decremented (p--)
>>>>>>>
>>>>>>> Yes, that's the cases what I want to handle, thanks for your explicit
>>>>>>> explanation.
>>>>>>>
>>>>>> Gave this some more thought over the weekend, and this implementation
>>>>>> doesn't solve the case when the last command times out and races with the
>>>>>> completion handler:
>>>>>>
>>>>>> cpu1                                    cpu2
>>>>>>
>>>>>> queue_command(first), p++ (=1)
>>>>>> --completion irq fires--                -- timer times out at same time--
>>>>>> handle_cmd_completion()                 handle_cmd_timeout(),)
>>>>>> lock(xhci_lock )                        spin_on(xhci_lock)
>>>>>> del_timer() fail, p (=1, nochange)
>>>>>> no more commands, P (=1, nochange)
>>>>>> unlock(xhci_lock)
>>>>>>                                          lock(xhci_lock)
>>>>>>                                          p-- (=0)
>>>>>>                                          p == 0, continue, even if we should
>>>>>> not.
>>>>>>                                            For this we still need to rely on
>>>>>> checking cur_cmd == NULL in the timeout function.
>>>>>> (Baolus patch sets it to NULL if there are no more commands pending)
>>>>> As I pointed out in patch 1 of this patchset, this patchset is based
>>>>> on Lu Baolu's new fix patch:
>>>>> usb: xhci: fix possible wild pointer
>>>>> https://www.spinics.net/lists/linux-usb/msg150219.html
>>>>>
>>>>> After applying Baolu's patch, after decrement the counter, we will
>>>>> check the xhci->cur_command if is NULL. So in this situation:
>>>>> cpu1                                    cpu2
>>>>>
>>>>>   queue_command(first), p++ (=1)
>>>>>   --completion irq fires--                -- timer times out at same time--
>>>>>   handle_cmd_completion()                 handle_cmd_timeout(),)
>>>>>   lock(xhci_lock )                        spin_on(xhci_lock)
>>>>>   del_timer() fail, p (=1, nochange)
>>>>>   no more commands, P (=1, nochange)
>>>>>   unlock(xhci_lock)
>>>>>                                           lock(xhci_lock)
>>>>>                                           p-- (=0)
>>>>>                                           no current command, return
>>>>>                                           if (!xhci->current_cmd) {
>>>>>                                                unlock(xhci_lock);
>>>>>                                                return;
>>>>>                                           }
>>>>>
>>>>> It can work.
>>>> Yes,
>>>>
>>>> What I wanted to say is that as it relies on Baolus patch for that one case
>>>> it seems that patch 2/2 can be replaced by a single line change:
>>>>
>>>> if (!xhci->current_cmd || timer_pending(&xhci->cmd_timer))
>>>>
>>>> Right?
>>>>
>>>> -Mathias
>>>>
>>> It seems that the watch dog algorithm for command queue becomes
>>> more and more complicated and hard for maintain. I am also seeing
>>> another case where a command may lose the chance to be tracked by
>>> the watch dog timer.
>>>
>>> Say,
>>>
>>> queue_command(the only command in queue)
>>>   - completion irq fires--                - timer times out at same time--      - another command enqueue--
>>>   - lock(xhci_lock )                         - spin_on(xhci_lock)                           - spin_on(xhci_lock)
>>>   - del_timer() fail
>>>   - free the command and
>>>     set current_cmd to NULL
>>>   - unlock(xhci_lock)
>>>                                                                                                                 - lock(xhci_lock)
>>>                                                                                                                 - queue_command()(timer will
>>>                                                                                                                    not rescheduled since the timer
>>>                                                                                                                    is pending)
>> In this case, since the command timer was fired and you did not re-add
>> the command timer, why here timer is pending? Maybe I missed
>> something? Thanks.
>
> In queue_command(),
>
>         /* if there are no other commands queued we start the timeout timer */
>         if (list_is_singular(&xhci->cmd_list) &&
>             !timer_pending(&xhci->cmd_timer)) {
>                 xhci->current_cmd = cmd;
>                 mod_timer(&xhci->cmd_timer, jiffies + XHCI_CMD_DEFAULT_TIMEOUT);
>         }
>
> timer_pending() will return true if the timer is fired, but the function is still
> running on another CPU. Do I understand it right?

>From my understanding, if the timer was fired, no matter the timeout
function is running or finished, timer_pending() will return false.
Please correct me if I made mistakes. Thanks.

>
> Best regards,
> Lu Baolu
>
>>>                                                      - lock(xhci_lock)
>>>                                                      - no current command
>>>                                                      - return
>>>
>>> As the result, the later command isn't under track of the watch dog.
>>> If hardware fails to response to this command, kernel will hang in
>>> the thread which is waiting for the completion of the command.
>>>
>>> I can write a patch to fix this and cc stable kernel as well. For long
>>> term, in order to make it simple and easy to maintain, how about
>>> allocating a watch dog timer for each command? It could be part
>>> of the command structure and be managed just like the life cycle
>>> of a command structure.
>>>
>>> I can write a patch for review and discussion, if you think this
>>> change is possible.
>>>
>>> Best regards,
>>> Lu Baolu
>>
>>
>



-- 
Baolin.wang
Best Regards