Re: [FFmpeg-devel] [PATCH 2/2] lavu/hwcontext_qsv: fix a potential infinite loop

[James Almer <jamrial@gmail.com>]

On 2/8/2022 3:26 PM, Anton Khirnov wrote:
> Current code will loop forever if MFXVideoVPP_Init() fails.
> Also, simplify the code.
> ---
>   libavutil/hwcontext_qsv.c | 80 +++++++++++++++------------------------
>   1 file changed, 31 insertions(+), 49 deletions(-)
> 
> diff --git a/libavutil/hwcontext_qsv.c b/libavutil/hwcontext_qsv.c
> index d3d8f42c99..95f8071abe 100644
> --- a/libavutil/hwcontext_qsv.c
> +++ b/libavutil/hwcontext_qsv.c
> @@ -16,6 +16,7 @@
>    * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>    */
>   
> +#include <stdatomic.h>
>   #include <stdint.h>
>   #include <string.h>
>   
> @@ -71,12 +72,11 @@ typedef struct QSVDeviceContext {
>   
>   typedef struct QSVFramesContext {
>       mfxSession session_download;
> -    int session_download_init;
> +    atomic_int session_download_init;
>       mfxSession session_upload;
> -    int session_upload_init;
> +    atomic_int session_upload_init;
>   #if HAVE_PTHREADS
>       pthread_mutex_t session_lock;
> -    pthread_cond_t session_cond;
>   #endif
>   
>       AVBufferRef *child_frames_ref;
> @@ -297,7 +297,6 @@ static void qsv_frames_uninit(AVHWFramesContext *ctx)
>   
>   #if HAVE_PTHREADS
>       pthread_mutex_destroy(&s->session_lock);
> -    pthread_cond_destroy(&s->session_cond);
>   #endif
>   
>       av_freep(&s->mem_ids);
> @@ -744,7 +743,6 @@ static int qsv_frames_init(AVHWFramesContext *ctx)
>   
>   #if HAVE_PTHREADS
>       pthread_mutex_init(&s->session_lock, NULL);
> -    pthread_cond_init(&s->session_cond, NULL);
>   #endif
>   
>       return 0;
> @@ -1024,6 +1022,32 @@ static int map_frame_to_surface(const AVFrame *frame, mfxFrameSurface1 *surface)
>       return 0;
>   }
>   
> +static int qsv_internal_session_check_init(AVHWFramesContext *ctx, int upload)
> +{
> +    QSVFramesContext *s = ctx->internal->priv;
> +    atomic_int *inited  = upload ? &s->session_upload_init : &s->session_download_init;
> +    mfxSession *session = upload ? &s->session_upload      : &s->session_download;
> +    int ret = 0;
> +
> +    if (atomic_load(inited))
> +        return 0;
> +
> +#if HAVE_PTHREADS
> +    pthread_mutex_lock(&s->session_lock);
> +#endif
> +
> +    if (!atomic_load(inited)) {
> +        ret = qsv_init_internal_session(ctx, session, upload);
> +        atomic_store(inited, 1);

Nit: How about

if (atomic_compare_exchange_strong(inited, &ret, 1))
     ret = qsv_init_internal_session(ctx, session, upload);

Or using some other variable for the second argument (It must be 0 
before the call, and will become 1 if *inited was already 1).

Since atomic_compare_exchange_strong() is not used anywhere in the tree, 
it would let us know if any of the atomics emulation implementations are 
faulty (win32, pthreads, old gcc).

> +    }
> +
> +#if HAVE_PTHREADS
> +    pthread_mutex_unlock(&s->session_lock);
> +#endif
> +
> +    return ret;
> +}
> +
>   static int qsv_transfer_data_from(AVHWFramesContext *ctx, AVFrame *dst,
>                                     const AVFrame *src)
>   {
> @@ -1035,28 +1059,7 @@ static int qsv_transfer_data_from(AVHWFramesContext *ctx, AVFrame *dst,
>       mfxStatus err;
>       int ret = 0;
>   
> -    while (!s->session_download_init && !s->session_download && !ret) {
> -#if HAVE_PTHREADS
> -        if (pthread_mutex_trylock(&s->session_lock) == 0) {
> -#endif
> -            if (!s->session_download_init) {
> -                ret = qsv_init_internal_session(ctx, &s->session_download, 0);
> -                if (s->session_download)
> -                    s->session_download_init = 1;
> -            }
> -#if HAVE_PTHREADS
> -            pthread_mutex_unlock(&s->session_lock);
> -            pthread_cond_signal(&s->session_cond);
> -        } else {
> -            pthread_mutex_lock(&s->session_lock);
> -            while (!s->session_download_init && !s->session_download) {
> -                pthread_cond_wait(&s->session_cond, &s->session_lock);
> -            }
> -            pthread_mutex_unlock(&s->session_lock);
> -        }
> -#endif
> -    }
> -
> +    ret = qsv_internal_session_check_init(ctx, 0);
>       if (ret < 0)
>           return ret;
>   
> @@ -1109,28 +1112,7 @@ static int qsv_transfer_data_to(AVHWFramesContext *ctx, AVFrame *dst,
>       const AVFrame *src_frame;
>       int realigned = 0;
>   
> -
> -    while (!s->session_upload_init && !s->session_upload && !ret) {
> -#if HAVE_PTHREADS
> -        if (pthread_mutex_trylock(&s->session_lock) == 0) {
> -#endif
> -            if (!s->session_upload_init) {
> -                ret = qsv_init_internal_session(ctx, &s->session_upload, 1);
> -                if (s->session_upload)
> -                    s->session_upload_init = 1;
> -            }
> -#if HAVE_PTHREADS
> -            pthread_mutex_unlock(&s->session_lock);
> -            pthread_cond_signal(&s->session_cond);
> -        } else {
> -            pthread_mutex_lock(&s->session_lock);
> -            while (!s->session_upload_init && !s->session_upload) {
> -                pthread_cond_wait(&s->session_cond, &s->session_lock);
> -            }
> -            pthread_mutex_unlock(&s->session_lock);
> -        }
> -#endif
> -    }
> +    ret = qsv_internal_session_check_init(ctx, 1);
>       if (ret < 0)
>           return ret;
>   
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