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

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Date:   Tue, 03 Jan 2017 14:33:23 +0200
From:   Felipe Balbi <balbi@...nel.org>
To:     Baolin Wang <baolin.wang@...aro.org>,
        Janusz Dziedzic <janusz.dziedzic@...il.com>
Cc:     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


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.

-- 
balbi

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