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Date: Tue, 20 Dec 2016 14:39:45 +0800
From: Lu Baolu <baolu.lu@...ux.intel.com>
To: Baolin Wang <baolin.wang@...aro.org>
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 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?
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
>
>
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