linux-kernel - Re: [PATCH v2] drm/msm/dp: fix event thread stuck in wait_event after kthread

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Message-ID: <b96858a9-c38f-6576-e61c-9a94b0ee53f1@quicinc.com>
Date:   Tue, 3 May 2022 09:13:02 -0700
From:   Kuogee Hsieh <quic_khsieh@...cinc.com>
To:     Stephen Boyd <swboyd@...omium.org>, <agross@...nel.org>,
        <airlied@...ux.ie>, <bjorn.andersson@...aro.org>,
        <daniel@...ll.ch>, <dianders@...omium.org>,
        <dmitry.baryshkov@...aro.org>, <robdclark@...il.com>,
        <sean@...rly.run>, <vkoul@...nel.org>
CC:     <quic_abhinavk@...cinc.com>, <quic_aravindh@...cinc.com>,
        <quic_sbillaka@...cinc.com>, <freedreno@...ts.freedesktop.org>,
        <dri-devel@...ts.freedesktop.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drm/msm/dp: fix event thread stuck in wait_event after
 kthread_stop()


On 5/2/2022 6:13 PM, Stephen Boyd wrote:
> Quoting Kuogee Hsieh (2022-05-02 16:04:28)
>> Event thread supposed to exit from its while loop after kthread_stop().
>> However there may has possibility that event thread is pending in the
>> middle of wait_event due to condition checking never become true.
>> To make sure event thread exit its loop after kthread_stop(), this
>> patch OR kthread_should_stop() into wait_event's condition checking
>> so that event thread will exit its loop after kernal_stop().
>>
>> Changes in v2:
>> --  correct spelling error at commit title
>>
>> Reported-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>> Fixes: 570d3e5d28db ("drm/msm/dp: stop event kernel thread when DP unbind")
>> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
>> ---
>>   drivers/gpu/drm/msm/dp/dp_display.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
>> index c388323..5200a58 100644
>> --- a/drivers/gpu/drm/msm/dp/dp_display.c
>> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
>> @@ -1106,12 +1106,17 @@ static int hpd_event_thread(void *data)
>>          while (!kthread_should_stop()) {
>>                  if (timeout_mode) {
>>                          wait_event_timeout(dp_priv->event_q,
>> -                               (dp_priv->event_pndx == dp_priv->event_gndx),
>> -                                               EVENT_TIMEOUT);
>> +                               ((dp_priv->event_pndx == dp_priv->event_gndx) ||
> Why the parenthesis (before and after)?
>
>> +                                       kthread_should_stop()), EVENT_TIMEOUT);
>>                  } else {
>>                          wait_event_interruptible(dp_priv->event_q,
>> -                               (dp_priv->event_pndx != dp_priv->event_gndx));
>> +                               ((dp_priv->event_pndx != dp_priv->event_gndx) ||
> Why the parenthesis (before and after)?
>
>> +                                       kthread_should_stop()));
>>                  }
>> +
>> +               if(kthread_should_stop())
> Missing space after if
>
>> +                       break;
> Is it possible to move the wait_event to the end of the loop and always
> run the loop initially? That way we don't have to check for
> kthread_should_stop() again.

no, since we have to make sure q is not empty so that we can proceed to 
service events.

bu ti thin we can use while (1) instead of while 
(!kthread_should_stop()) since we have add kthread_should_stop() into 
wait_event condition checking.