Re: [FFmpeg-devel] [PATCH] lavc/pthread_frame: avoid leaving stale hwaccel state in worker threads

[Steve Lhomme <robux4@ycbcr.xyz>]

Hi Anton,

On 2022-09-02 22:59, Anton Khirnov wrote:
> This state is not refcounted, so make sure it always has a well-defined
> owner.
> ---
> Steve, could you please test this?

I can confirm it doesn't leak the context and plays correctly. It also 
doesn't crash ;)

> ---
>   libavcodec/pthread_frame.c | 37 ++++++++++++++++++++++++++++++++-----
>   1 file changed, 32 insertions(+), 5 deletions(-)
> 
> diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
> index 08a6f98898..9b44e2e698 100644
> --- a/libavcodec/pthread_frame.c
> +++ b/libavcodec/pthread_frame.c
> @@ -148,6 +148,10 @@ typedef struct FrameThreadContext {
>                                       * Set for the first N packets, where N is the number of threads.
>                                       * While it is set, ff_thread_en/decode_frame won't return any results.
>                                       */
> +
> +    const AVHWAccel *stash_hwaccel;
> +    void            *stash_hwaccel_context;
> +    void            *stash_hwaccel_priv;
>   } FrameThreadContext;
>   
>   #if FF_API_THREAD_SAFE_CALLBACKS
> @@ -228,9 +232,17 @@ FF_ENABLE_DEPRECATION_WARNINGS
>               ff_thread_finish_setup(avctx);
>   
>           if (p->hwaccel_serializing) {
> +            /* wipe hwaccel state to avoid stale pointers lying around;
> +             * the state was transferred to FrameThreadContext in
> +             * ff_thread_finish_setup(), so nothing is leaked */
> +            avctx->hwaccel                     = NULL;
> +            avctx->hwaccel_context             = NULL;
> +            avctx->internal->hwaccel_priv_data = NULL;
> +
>               p->hwaccel_serializing = 0;
>               pthread_mutex_unlock(&p->parent->hwaccel_mutex);
>           }
> +        av_assert0(!avctx->hwaccel);
>   
>           if (p->async_serializing) {
>               p->async_serializing = 0;
> @@ -294,9 +306,6 @@ static int update_context_from_thread(AVCodecContext *dst, AVCodecContext *src,
>           dst->color_range = src->color_range;
>           dst->chroma_sample_location = src->chroma_sample_location;
>   
> -        dst->hwaccel = src->hwaccel;
> -        dst->hwaccel_context = src->hwaccel_context;
> -
>           dst->sample_rate    = src->sample_rate;
>           dst->sample_fmt     = src->sample_fmt;
>   #if FF_API_OLD_CHANNEL_LAYOUT
> @@ -309,8 +318,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
>           if (err < 0)
>               return err;
>   
> -        dst->internal->hwaccel_priv_data = src->internal->hwaccel_priv_data;
> -
>           if (!!dst->hw_frames_ctx != !!src->hw_frames_ctx ||
>               (dst->hw_frames_ctx && dst->hw_frames_ctx->data != src->hw_frames_ctx->data)) {
>               av_buffer_unref(&dst->hw_frames_ctx);
> @@ -450,6 +457,12 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
>               pthread_mutex_unlock(&p->mutex);
>               return err;
>           }
> +
> +        /* transfer hwaccel state stashed from previous thread, if any */
> +        av_assert0(!p->avctx->hwaccel);
> +        FFSWAP(const AVHWAccel*, p->avctx->hwaccel,                     fctx->stash_hwaccel);
> +        FFSWAP(void*,            p->avctx->hwaccel_context,             fctx->stash_hwaccel_context);
> +        FFSWAP(void*,            p->avctx->internal->hwaccel_priv_data, fctx->stash_hwaccel_priv);
>       }
>   
>       av_packet_unref(p->avpkt);
> @@ -655,6 +668,13 @@ void ff_thread_finish_setup(AVCodecContext *avctx) {
>           async_lock(p->parent);
>       }
>   
> +    /* save hwaccel state for passing to the next thread;
> +     * this is done here so that this worker thread can wipe its own hwaccel
> +     * state after decoding, without requiring synchronization */
> +    p->parent->stash_hwaccel         = avctx->hwaccel;
> +    p->parent->stash_hwaccel_context = avctx->hwaccel_context;
> +    p->parent->stash_hwaccel_priv    = avctx->internal->hwaccel_priv_data;
> +
>       pthread_mutex_lock(&p->progress_mutex);
>       if(atomic_load(&p->state) == STATE_SETUP_FINISHED){
>           av_log(avctx, AV_LOG_WARNING, "Multiple ff_thread_finish_setup() calls\n");
> @@ -761,6 +781,13 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
>       av_freep(&fctx->threads);
>       ff_pthread_free(fctx, thread_ctx_offsets);
>   
> +    /* if we have stashed hwaccel state, move it to the user-facing context,
> +     * so it will be freed in avcodec_close() */
> +    av_assert0(!avctx->hwaccel);
> +    FFSWAP(const AVHWAccel*, avctx->hwaccel,                     fctx->stash_hwaccel);
> +    FFSWAP(void*,            avctx->hwaccel_context,             fctx->stash_hwaccel_context);
> +    FFSWAP(void*,            avctx->internal->hwaccel_priv_data, fctx->stash_hwaccel_priv);
> +
>       av_freep(&avctx->internal->thread_ctx);
>   }
>   
> -- 
> 2.35.1
> 
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