[FFmpeg-devel] [PATCH] frame_thread_encoder: make task indexing deterministic.

[Ronald S. Bultje]

Fixes tsan warnings in fate-utvideoenc. Example warning from
utvideoenc_rgb_left:

WARNING: ThreadSanitizer: data race (pid=19929)
  Read of size 8 at 0x7d840001cf18 by main thread:
    #0 ff_thread_video_encode_frame src/libavcodec/frame_thread_encoder.c:276 (ffmpeg+0x00000136a39d)
    #1 avcodec_encode_video2 src/libavcodec/utils.c:1986 (ffmpeg+0x000000f34a18)
[..]
  Previous write of size 8 at 0x7d840001cf18 by thread T14 (mutexes: write M1348):
    #0 worker src/libavcodec/frame_thread_encoder.c:100 (ffmpeg+0x000001369964)

We prevent that by using the same mechanism as frame-mt decoding, and
assuming that we're encoding N packets in parallel (where N=n_threads),
and thus the first N calls to encode_video() won't produce a packet.
---
 libavcodec/frame_thread_encoder.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/libavcodec/frame_thread_encoder.c b/libavcodec/frame_thread_encoder.c
index 27ae356..8db9290 100644
--- a/libavcodec/frame_thread_encoder.c
+++ b/libavcodec/frame_thread_encoder.c
@@ -251,6 +251,7 @@ int ff_thread_video_encode_frame(AVCodecContext *avctx, AVPacket *pkt, const AVF
     ThreadContext *c = avctx->internal->frame_thread_encoder;
     Task task;
     int ret;
+    unsigned idx;
 
     av_assert1(!*got_packet_ptr);
 
@@ -264,32 +265,33 @@ int ff_thread_video_encode_frame(AVCodecContext *avctx, AVPacket *pkt, const AVF
             return ret;
         }
 
-        task.index = c->task_index;
+        task.index = c->task_index % BUFFER_SIZE;
         task.indata = (void*)new;
         pthread_mutex_lock(&c->task_fifo_mutex);
         av_fifo_generic_write(c->task_fifo, &task, sizeof(task), NULL);
         pthread_cond_signal(&c->task_fifo_cond);
         pthread_mutex_unlock(&c->task_fifo_mutex);
 
-        c->task_index = (c->task_index+1) % BUFFER_SIZE;
+        c->task_index++;
 
-        if(!c->finished_tasks[c->finished_task_index].outdata && (c->task_index - c->finished_task_index) % BUFFER_SIZE <= avctx->thread_count)
+        if (c->task_index - c->finished_task_index < avctx->thread_count)
             return 0;
     }
 
     if(c->task_index == c->finished_task_index)
         return 0;
 
+    idx = c->finished_task_index % BUFFER_SIZE;
     pthread_mutex_lock(&c->finished_task_mutex);
-    while (!c->finished_tasks[c->finished_task_index].outdata) {
+    while (!c->finished_tasks[idx].outdata) {
         pthread_cond_wait(&c->finished_task_cond, &c->finished_task_mutex);
     }
-    task = c->finished_tasks[c->finished_task_index];
+    task = c->finished_tasks[idx];
     *pkt = *(AVPacket*)(task.outdata);
     if(pkt->data)
         *got_packet_ptr = 1;
-    av_freep(&c->finished_tasks[c->finished_task_index].outdata);
-    c->finished_task_index = (c->finished_task_index+1) % BUFFER_SIZE;
+    av_freep(&c->finished_tasks[idx].outdata);
+    c->finished_task_index++;
     pthread_mutex_unlock(&c->finished_task_mutex);
 
     return task.return_code;
-- 
2.8.1