linux-kernel - Re: [PATCH] usb: dwc3: gadget: Avoid race between dwc3 interrupt handler and irq thread handler

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Date:   Thu, 5 Jan 2017 10:07:22 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Felipe Balbi <balbi@...nel.org>
Cc:     Janusz Dziedzic <janusz.dziedzic@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] usb: dwc3: gadget: Avoid race between dwc3 interrupt
 handler and irq thread handler

On 3 January 2017 at 20:33, Felipe Balbi <balbi@...nel.org> wrote:
>
> Hi,
>
> Baolin Wang <baolin.wang@...aro.org> writes:
>> On 28 December 2016 at 20:30, Janusz Dziedzic <janusz.dziedzic@...il.com> wrote:
>>> 2016-12-27 13:16 GMT+01:00 Baolin Wang <baolin.wang@...aro.org>:
>>>> Hi,
>>>>
>>>> On 27 December 2016 at 19:11, Felipe Balbi <balbi@...nel.org> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> Baolin Wang <baolin.wang@...aro.org> writes:
>>>>>> Hi,
>>>>>>
>>>>>> On 27 December 2016 at 18:52, Janusz Dziedzic <janusz.dziedzic@...il.com> wrote:
>>>>>>> 2016-12-26 9:01 GMT+01:00 Baolin Wang <baolin.wang@...aro.org>:
>>>>>>>> On some platfroms(like x86 platform), when one core is running the USB gadget
>>>>>>>> irq thread handler by dwc3_thread_interrupt(), meanwhile another core also can
>>>>>>>> respond other interrupts from dwc3 controller and modify the event buffer by
>>>>>>>> dwc3_interrupt() function, that will cause getting the wrong event count in
>>>>>>>> irq thread handler to make the USB function abnormal.
>>>>>>>>
>>>>>>>> We should add spin_lock/unlock() in dwc3_check_event_buf() to avoid this race.
>>>>>>>>
>>>>>>> Interesting, I always think we mask interrupt in dwc3_interrupt() by setting
>>>>>>> DWC3_GEVNTSIZ_INTMASK
>>>>>>> And unmask interrupt when we end dwc3_thread_interrupt().
>>>>>>>
>>>>>>> So, we shouldn't get any IRQ from HW during dwc3_thread_interrupt(),
>>>>>>> or I miss something?
>>>>>>> Do you have some traces that indicate this masking will not work correctly?
>>>>>>
>>>>>> Yes, but we just masked the interrupts described in DEVTEN register,
>>>>>> and we did not mask all the interrupts, like the endpoint command
>>>>>> complete event, transfer complete event and so on, so we can still get
>>>>>> interrupts.
>>>>>
>>>>> not true, we masked interrupts for the entire event buffer:
>>>>
>>>> Yes, you are right and I missed that. I should reproduce this problem
>>>> and analyse the real reason.
>>>>
>>>>>
>>>>>> static irqreturn_t dwc3_check_event_buf(struct dwc3_event_buffer *evt)
>>>>>> {
>>>>>>       struct dwc3 *dwc = evt->dwc;
>>>>>>       u32 count;
>>>>>>       u32 reg;
>>>>>>
>>>>>>       if (pm_runtime_suspended(dwc->dev)) {
>>>>>>               pm_runtime_get(dwc->dev);
>>>>>>               disable_irq_nosync(dwc->irq_gadget);
>>>>>>               dwc->pending_events = true;
>>>>>>               return IRQ_HANDLED;
>>>>>>       }
>>>>>>
>>>>>>       count = dwc3_readl(dwc->regs, DWC3_GEVNTCOUNT(0));
>>>>>>       count &= DWC3_GEVNTCOUNT_MASK;
>>>>>>       if (!count)
>>>>>>               return IRQ_NONE;
>>>>>>
>>>>>>       evt->count = count;
>>>>>>       evt->flags |= DWC3_EVENT_PENDING;
>>>>>>
>>>>>>       /* Mask interrupt */
>>>>>>       reg = dwc3_readl(dwc->regs, DWC3_GEVNTSIZ(0));
>>>>>>       reg |= DWC3_GEVNTSIZ_INTMASK;
>>>>>
>>>>> See here ?!?
>>>>>
>>>>>>       dwc3_writel(dwc->regs, DWC3_GEVNTSIZ(0), reg);
>>>>>>
>>>>>>       return IRQ_WAKE_THREAD;
>>>>>> }
>>>>>
>>>>>>> BTW, what value you get when problem occured, 0xFFFC?
>>>>>>
>>>>>> Yes, something like this, the event count become huge.
>>>>>
>>> Probably you have little bit different code than current community
>>> version (depends how your PM works).
>>>
>>> This is possible when we write:
>>> dwc3_writel(dwc->regs, DWC3_GEVNTCOUNT(0), 0);
>>> And after that
>>> dwc3_writel(dwc->regs, DWC3_GEVNTCOUNT(0), 4);
>>>
>>> After that we will get 0xFFFC (-4).
>>>
>>> Possible races:
>>> 1) dwc3_event_buffers_setup/dwc3_event_buffers_cleanup - write 0
>>> 2) dwc3_thread - write 4
>>>
>>> While [1] could be called in PM work or UM context (init in Android
>>> case) spin_lock_irqsave() will only disable local irqs and still we
>>> could get IRQ on different core, next update evt->count and run
>>> thread...
>>
>> Yeah, that's the possible races.
>
> and you have triggered this with mailine? How? We don't write to GEVNT*
> registers from PM code and we only allow runtime_suspend with cable
> dettached.

Sorry for late reply since I am busy on other things. I just agreed
with the possible races pointed by Janusz. I need to look at if these
are happened on my platform and also I found some out of tree code
which will clean the GEVNTCOUNT register when stop the gadget. I will
check the mainline kernel and resend new patch if I make this problem
clearly. Anyway thanks for your help and suggestion.

-- 
Baolin.wang
Best Regards