Re: [FFmpeg-devel] [PATCH 31/92] Vulkan patchset part 1 - common code changes

[Lynne <dev@lynne.ee>]

Apr 17, 2023, 11:25 by anton@khirnov.net:

> Quoting Lynne (2023-03-14 07:33:43)
>
>> From 6e2dfc44e50798264eb16bc2dcabfdbf2fbac2d7 Mon Sep 17 00:00:00 2001
>> From: Lynne <dev@lynne.ee>
>> Date: Thu, 15 Dec 2022 01:06:52 +0100
>> Subject: [PATCH 24/92] hwconfig: add a new HWACCEL_CAP_THREAD_SAFE for
>>  threadsafe hwaccels
>>
>> Vulkan is fully threadsafe and stateless, so we can benefit from this.
>> ---
>>  libavcodec/hwconfig.h      | 1 +
>>  libavcodec/pthread_frame.c | 2 +-
>>  2 files changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavcodec/hwconfig.h b/libavcodec/hwconfig.h
>> index 721424912c..e6b78f0160 100644
>> --- a/libavcodec/hwconfig.h
>> +++ b/libavcodec/hwconfig.h
>> @@ -24,6 +24,7 @@
>>  
>>  
>>  #define HWACCEL_CAP_ASYNC_SAFE      (1 << 0)
>> +#define HWACCEL_CAP_THREAD_SAFE     (1 << 1)
>>  
>>  
>>  typedef struct AVCodecHWConfigInternal {
>> diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
>> index 74864e19c5..c096287233 100644
>> --- a/libavcodec/pthread_frame.c
>> +++ b/libavcodec/pthread_frame.c
>> @@ -204,7 +204,7 @@ static attribute_align_arg void *frame_worker_thread(void *arg)
>>  
>>  /* if the previous thread uses hwaccel then we take the lock to ensure
>>  * the threads don't run concurrently */
>> -        if (avctx->hwaccel) {
>> +        if (avctx->hwaccel && !(avctx->hwaccel->caps_internal & HWACCEL_CAP_THREAD_SAFE)) {
>>
>
> A major problem here is that hwaccel_priv_data exists in only a single
> instance shared across all the worker threads. If you want frame-threaded
> hwaccels, you'll need to add some mechanism for synchronizing it between
> threads.
>
> I really wonder how does your code survive all the races involved.
>

It works because hwaccel_priv_data is read-only. No need to synchronize
anything, the contexts Vulkan creates are defined as driver-synchronized,
and all the info a frame needs is provided when decoding it, rather than
kept as a state.
So I don't see a problem here.
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