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Date: Tue, 20 Dec 2016 15:30:59 +0800
From: Baolin Wang <baolin.wang@...aro.org>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Mathias Nyman <mathias.nyman@...el.com>,
Mathias Nyman <mathias.nyman@...ux.intel.com>,
Greg KH <gregkh@...uxfoundation.org>,
USB <linux-usb@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Mark Brown <broonie@...nel.org>,
"Lu, Baolu" <baolu.lu@...el.com>
Subject: Re: [PATCH 2/2] usb: host: xhci: Handle the right timeout command
Hi,
On 20 December 2016 at 15:18, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> Hi,
>
> On 12/20/2016 02:46 PM, Baolin Wang wrote:
>> On 20 December 2016 at 14:39, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>>> Hi,
>>>
>>> On 12/20/2016 02:06 PM, Baolin Wang wrote:
>>>> Hi,
>>>>
>>>> On 20 December 2016 at 12:29, Lu Baolu <baolu.lu@...ux.intel.com> 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@...ux.intel.com> wrote:
>>>>>>>> On 13.12.2016 05:21, Baolin Wang wrote:
>>>>>>>>> Hi Mathias,
>>>>>>>>>
>>>>>>>>> On 12 December 2016 at 23:52, Mathias Nyman
>>>>>>>>> <mathias.nyman@...ux.intel.com> 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.
>
> Just looked into kernel/time/timer.c. You are right.
>
> The pending is cleared in expire_timers() before call the timer function.
>
> Base on this fact, we don't need to check timer_pending() at all in below code.
Indeed, I think so too.
>
> /* if there are no other commands queued we start the timeout timer */
> if (xhci->cmd_list.next == &cmd->cmd_list &&
> !timer_pending(&xhci->cmd_timer)) {
> xhci->current_cmd = cmd;
> mod_timer(&xhci->cmd_timer, jiffies + XHCI_CMD_DEFAULT_TIMEOUT);
> }
>
>
> Best regards,
> Lu Baolu
>
>>
>>> 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
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