linux-kernel - Re: [PATCH v2 3/3] media: amphion: Remove lock in s

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Date: Tue, 30 Apr 2024 16:55:06 +0800
From: ming qian <ming.qian@....nxp.com>
To: Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
 Ming Qian <ming.qian@....com>, mchehab@...nel.org, hverkuil-cisco@...all.nl
Cc: shawnguo@...nel.org, robh+dt@...nel.org, s.hauer@...gutronix.de,
 kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
 xiahong.bao@....com, eagle.zhou@....com, tao.jiang_2@....com,
 imx@...ts.linux.dev, linux-media@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/3] media: amphion: Remove lock in s_ctrl callback


Hi Andrzej,

> Hi Ming,
> 
> W dniu 30.04.2024 o 04:32, ming qian pisze:
>> Hi Andrzej,
>>
>>> Hi Ming Qian,
>>>
>>> W dniu 25.04.2024 o 08:50, Ming Qian pisze:
>>>> There is no need to add lock in s_ctrl callback, it has been
>>>> synchronized by the ctrl_handler's lock, otherwise it may led to
>>>> deadlock if driver call v4l2_ctrl_s_ctrl().
>>>>
>>>> Signed-off-by: Ming Qian <ming.qian@....com>
>>>> ---
>>>>   drivers/media/platform/amphion/vdec.c | 2 --
>>>>   drivers/media/platform/amphion/venc.c | 2 --
>>>>   2 files changed, 4 deletions(-)
>>>>
>>>> diff --git a/drivers/media/platform/amphion/vdec.c 
>>>> b/drivers/media/platform/amphion/vdec.c
>>>> index a57f9f4f3b87..6a38a0fa0e2d 100644
>>>> --- a/drivers/media/platform/amphion/vdec.c
>>>> +++ b/drivers/media/platform/amphion/vdec.c
>>>> @@ -195,7 +195,6 @@ static int vdec_op_s_ctrl(struct v4l2_ctrl *ctrl)
>>>>       struct vdec_t *vdec = inst->priv;
>>>>       int ret = 0;
>>>> -    vpu_inst_lock(inst);
>>>
>>> I assume that PATCH v2 2/3 might cause the said deadlock to happen?
>>> If so, maybe it would make more sense to make the current patch preceed
>>>   PATCH v2 2/3? Otherwise the kernel at PATCH v2 2/3 introduces a 
>>> potential
>>> deadlock.
>>>
>>> Regards,
>>>
>>> Andrzej
>>>
>>
>> I actually discovered this problem when I was preparing the v2 2/3 patch.
>>
>> But in the v2 2/3 patch, it tried to add a read-only ctrl, then I just
>> unset the s_ctrl callback for the new added ctrl, the potential deadlock
>> is caused by call the s_ctrl back in a locked environment, so after unset
>> the s_ctrl callback, the 2/3 patch won't trigger the deadlock even if
>> this patch is missing.
>>
>> In order to avoid encountering similar problems in the future, that
>> driver may set or get some ctrl, I added this patch.
> 
> Do I understand you correctly that patch 2/3 is written in such a way that
> it does not introduce a deadlock, and you add patch 3/3 only to prevent 
> future
> problems? If so, it seems to me that patch 3/3 could/should be separate 
> from
> this series, as it does not quite match "Add average qp control".
> 
> Regards,
> 
> Andrzej
> 

You're right, when I prepare the 2/3 patch, I add the new ctrl just like
the previous ctrl, but I got a deadlock. then I tried to fix a deadlock
in 2/3 patch first, and make this patch again to avoid similar errors.

I put this patch to this series, as I think this better explains why I
made this change.

Regards,
Ming

>>
>> Best regards,
>> Ming
>>
>>>>       switch (ctrl->id) {
>>>>       case V4L2_CID_MPEG_VIDEO_DEC_DISPLAY_DELAY_ENABLE:
>>>>           vdec->params.display_delay_enable = ctrl->val;
>>>> @@ -207,7 +206,6 @@ static int vdec_op_s_ctrl(struct v4l2_ctrl *ctrl)
>>>>           ret = -EINVAL;
>>>>           break;
>>>>       }
>>>> -    vpu_inst_unlock(inst);
>>>>       return ret;
>>>>   }
>>>> diff --git a/drivers/media/platform/amphion/venc.c 
>>>> b/drivers/media/platform/amphion/venc.c
>>>> index cdfaba9d107b..351b4edc8742 100644
>>>> --- a/drivers/media/platform/amphion/venc.c
>>>> +++ b/drivers/media/platform/amphion/venc.c
>>>> @@ -518,7 +518,6 @@ static int venc_op_s_ctrl(struct v4l2_ctrl *ctrl)
>>>>       struct venc_t *venc = inst->priv;
>>>>       int ret = 0;
>>>> -    vpu_inst_lock(inst);
>>>>       switch (ctrl->id) {
>>>>       case V4L2_CID_MPEG_VIDEO_H264_PROFILE:
>>>>           venc->params.profile = ctrl->val;
>>>> @@ -579,7 +578,6 @@ static int venc_op_s_ctrl(struct v4l2_ctrl *ctrl)
>>>>           ret = -EINVAL;
>>>>           break;
>>>>       }
>>>> -    vpu_inst_unlock(inst);
>>>>       return ret;
>>>>   }
>>>
>