linux-kernel - Re: [PATCH v9 06/13] media: atmel: atmel-isc-base: use mutex to lock awb workqueue from streaming

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Message-ID: <2510d479-5eef-6ef6-5d32-41f70217eb52@microchip.com>
Date:   Fri, 29 Apr 2022 08:08:45 +0000
From:   <Eugen.Hristev@...rochip.com>
To:     <hverkuil@...all.nl>, <linux-media@...r.kernel.org>,
        <jacopo@...ndi.org>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <Claudiu.Beznea@...rochip.com>, <robh+dt@...nel.org>,
        <Nicolas.Ferre@...rochip.com>
Subject: Re: [PATCH v9 06/13] media: atmel: atmel-isc-base: use mutex to lock
 awb workqueue from streaming

On 4/29/22 10:41 AM, Hans Verkuil wrote:
> Hi Eugen,
> 
> On 10/03/2022 10:51, Eugen Hristev wrote:
>> The AWB workqueue runs in a kernel thread and needs to be synchronized
>> w.r.t. the streaming status.
>> It is possible that streaming is stopped while the AWB workq is running.
>> In this case it is likely that the check for vb2_start_streaming_called is done
>> at one point in time, but the AWB computations are done later, including a call
>> to isc_update_profile, which requires streaming to be started.
>> Thus , isc_update_profile will fail if during this operation sequence the
>> streaming was stopped.
>> To solve this issue, a mutex is added, that will serialize the awb work and
>> streaming stopping, with the mention that either streaming is stopped
>> completely including termination of the last frame is done, and after that
>> the AWB work can check stream status and stop; either first AWB work is
>> completed and after that the streaming can stop correctly.
>> The awb spin lock cannot be used since this spinlock is taken in the same
>> context and using it in the stop streaming will result in a recursion BUG.
> 
> Please keep the line length in a commit log to no more than 75.
> 
>>
>> Signed-off-by: Eugen Hristev <eugen.hristev@...rochip.com>
>> Reviewed-by: Jacopo Mondi <jacopo@...ndi.org>
>> ---
>>   drivers/media/platform/atmel/atmel-isc-base.c | 29 ++++++++++++++++---
>>   drivers/media/platform/atmel/atmel-isc.h      |  2 ++
>>   2 files changed, 27 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/media/platform/atmel/atmel-isc-base.c b/drivers/media/platform/atmel/atmel-isc-base.c
> 
> <snip>
> 
>> @@ -1548,6 +1564,7 @@ static int isc_s_awb_ctrl(struct v4l2_ctrl *ctrl)
>>                         */
>>                        v4l2_ctrl_activate(isc->do_wb_ctrl, false);
>>                }
>> +             mutex_unlock(&isc->awb_mutex);
> 
> Huh? What is this unlock doing here? Am I missing something?

Hello Hans,

Sorry. It looks like the corresponding mutex_lock was lost when I 
rebased the series over the patch to use 'is_streaming' instead of 
'stop' variable.
In version 3, the mutex_lock was there :

https://www.spinics.net/lists/linux-media/msg204059.html

Somehow a race problem did not occur in this specific critical area in 
my tests.

I will resend this patch as a v10 , or the whole series if you have 
other comments on the other patches? How would you prefer ?

Thanks,
Eugen

> 
> Regards,
> 
>          Hans
> 
>>
>>                /* if we have autowhitebalance on, start histogram procedure */
>>                if (ctrls->awb == ISC_WB_AUTO &&
>> @@ -1740,6 +1757,7 @@ static void isc_async_unbind(struct v4l2_async_notifier *notifier,
>>   {
>>        struct isc_device *isc = container_of(notifier->v4l2_dev,
>>                                              struct isc_device, v4l2_dev);
>> +     mutex_destroy(&isc->awb_mutex);
>>        cancel_work_sync(&isc->awb_work);
>>        video_unregister_device(&isc->video_dev);
>>        v4l2_ctrl_handler_free(&isc->ctrls.handler);
>> @@ -1850,6 +1868,8 @@ static int isc_async_complete(struct v4l2_async_notifier *notifier)
>>        isc->current_subdev = container_of(notifier,
>>                                           struct isc_subdev_entity, notifier);
>>        mutex_init(&isc->lock);
>> +     mutex_init(&isc->awb_mutex);
>> +
>>        init_completion(&isc->comp);
>>
>>        /* Initialize videobuf2 queue */
>> @@ -1930,6 +1950,7 @@ static int isc_async_complete(struct v4l2_async_notifier *notifier)
>>        video_unregister_device(vdev);
>>
>>   isc_async_complete_err:
>> +     mutex_destroy(&isc->awb_mutex);
>>        mutex_destroy(&isc->lock);
>>        return ret;
>>   }
>> diff --git a/drivers/media/platform/atmel/atmel-isc.h b/drivers/media/platform/atmel/atmel-isc.h
>> index 9cc69c3ae26d..f98f25a55e73 100644
>> --- a/drivers/media/platform/atmel/atmel-isc.h
>> +++ b/drivers/media/platform/atmel/atmel-isc.h
>> @@ -229,6 +229,7 @@ enum isc_scaler_pads {
>>    *
>>    * @lock:            lock for serializing userspace file operations
>>    *                   with ISC operations
>> + * @awb_mutex:               serialize access to streaming status from awb work queue
>>    * @awb_lock:                lock for serializing awb work queue operations
>>    *                   with DMA/buffer operations
>>    *
>> @@ -307,6 +308,7 @@ struct isc_device {
>>        struct work_struct      awb_work;
>>
>>        struct mutex            lock;
>> +     struct mutex            awb_mutex;
>>        spinlock_t              awb_lock;
>>
>>        struct regmap_field     *pipeline[ISC_PIPE_LINE_NODE_NUM];
>