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

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Message-ID: <0c0e2d02-b719-4f8b-afca-c839bb77f287@collabora.com>
Date: Tue, 30 Apr 2024 10:00:39 +0200
From: Andrzej Pietrasiewicz <andrzej.p@...labora.com>
To: ming qian <ming.qian@....nxp.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 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

> 
> 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;
>>>   }
>>