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

[Mathias Nyman <mathias.nyman@intel.com>]

On 19.12.2016 13:34, Baolin Wang wrote:
> Hi Mathias,
>
> On 19 December 2016 at 18:33, Mathias Nyman
> <mathias.nyman@linux.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@linux.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