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

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Message-ID: <be54f273-7bba-4db0-bc52-5ddbb3982d84@oss.nxp.com>
Date: Tue, 30 Apr 2024 10:32:58 +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 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.

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;
>>   }
>