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Date: Tue, 20 Dec 2016 11:23:29 +0800
From: Baolin Wang <baolin.wang@...aro.org>
To: Mathias Nyman <mathias.nyman@...el.com>
Cc: Mathias Nyman <mathias.nyman@...ux.intel.com>,
Greg KH <gregkh@...uxfoundation.org>,
Lu Baolu <baolu.lu@...ux.intel.com>,
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
On 19 December 2016 at 20:13, Mathias Nyman <mathias.nyman@...el.com> 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?
After checking the code again, I am agree with you. I will resend the
patch with fixing this issue. Thanks.
--
Baolin.wang
Best Regards
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