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

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Date:   Wed, 28 Dec 2016 13:30:14 +0100
From:   Janusz Dziedzic <janusz.dziedzic@...il.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Felipe Balbi <balbi@...nel.org>,
        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

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...

So, seems your patch will solve this.

I am not sure this problem could be also visible in community current version.

Anyway, thanks for handling this.

BR
Janusz
>> please send us tracepoint data. You probably need to compress
>> it. Something like 256k of trace data is probably enough, so:
>>
>> # mkdir -p /t
>> # mount -t tracefs none /t
>> # cd /t
>> # echo 256 > buffer_size_kb
>> # echo 1 > events/dwc3/enable
>> # echo 0 > events/dwc3/dwc3_readl/enable
>> # echo 0 > events/dwc3/dwc3_writel/enable
>>
>> (reproduce)
>>
>> # cp /t/trace /path/to/non-volatile/media/trace.txt
>
> Okay, I try to do that. Thanks.
>
> --
> Baolin.wang
> Best Regards



-- 
Janusz Dziedzic