linux-kernel - Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Mon, 6 Apr 2020 14:18:18 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>, <hverkuil@...all.nl>,
        <sakari.ailus@....fi>, <helen.koike@...labora.com>
CC:     <sboyd@...nel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver


On 4/6/20 1:54 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 06.04.2020 23:38, Sowjanya Komatineni пишет:
>> On 4/6/20 1:37 PM, Dmitry Osipenko wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 06.04.2020 23:20, Sowjanya Komatineni пишет:
>>>> On 4/6/20 1:02 PM, Dmitry Osipenko wrote:
>>>>> External email: Use caution opening links or attachments
>>>>>
>>>>>
>>>>> 04.04.2020 04:25, Sowjanya Komatineni пишет:
>>>>> ...
>>>>>> +static int chan_capture_kthread_start(void *data)
>>>>>> +{
>>>>>> +     struct tegra_vi_channel *chan = data;
>>>>>> +     struct tegra_channel_buffer *buf;
>>>>>> +     int err = 0;
>>>>>> +     int caps_inflight;
>>>>>> +
>>>>>> +     set_freezable();
>>>>>> +
>>>>>> +     while (1) {
>>>>>> +             try_to_freeze();
>>>>>> +
>>>>>> +             wait_event_interruptible(chan->start_wait,
>>>>>> +                                      !list_empty(&chan->capture) ||
>>>>>> +                                      kthread_should_stop());
>>>>> Is it really okay that list_empty() isn't protected with a lock?
>>>>>
>>>>> Why wait_event is "interruptible"?
>>>> To allow it to sleep until wakeup on thread it to avoid constant
>>>> checking for condition even when no buffers are ready, basically to
>>>> prevent blocking.
>>> So the "interrupt" is for getting event about kthread_should_stop(),
>>> correct?
>> also to prevent blocking and to let is sleep and wakeup based on wait
>> queue to evaluate condition to proceed with the task
> This looks suspicious, the comment to wait_event_interruptible() says
> that it will return ERESTARTSYS if signal is recieved..
>
> Does this mean that I can send signal from userspace to wake it up?
>
> The "interruptible" part looks wrong to me.

We are not checking for wait_event_interruptible to handle case when it 
returns ERESTARTSYS.

So, signals sent from user space are ignore and we check if when wakeup 
happens if kthread_stop has requested to stop thread.