[FFmpeg-devel] [PATCH] pthread_frame: make accesses to debug field be protected by owner lock.

[Ronald S. Bultje]

The av_log() is done outside the lock, but this way the accesses to the
field (reads and writes) are always protected by a mutex. The av_log()
is not run inside the lock context because it may involve user callbacks
and doing that in performance-sensitive code is probably not a good idea.

This should fix occasional tsan warnings when running fate-h264, like:

WARNING: ThreadSanitizer: data race (pid=10916)
  Write of size 4 at 0x7d64000174fc by main thread (mutexes: write M2313):
    #0 update_context_from_user src/libavcodec/pthread_frame.c:335 (ffmpeg+0x000000df7b06)
[..]
  Previous read of size 4 at 0x7d64000174fc by thread T1 (mutexes: write M2311):
    #0 ff_thread_await_progress src/libavcodec/pthread_frame.c:592 (ffmpeg+0x000000df8b3e)
---
 libavcodec/pthread_frame.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
index c246c2f..8857bfc 100644
--- a/libavcodec/pthread_frame.c
+++ b/libavcodec/pthread_frame.c
@@ -559,6 +559,7 @@ void ff_thread_report_progress(ThreadFrame *f, int n, int field)
 {
     PerThreadContext *p;
     atomic_int *progress = f->progress ? (atomic_int*)f->progress->data : NULL;
+    int debug_mv;
 
     if (!progress ||
         atomic_load_explicit(&progress[field], memory_order_relaxed) >= n)
@@ -566,22 +567,24 @@ void ff_thread_report_progress(ThreadFrame *f, int n, int field)
 
     p = f->owner[field]->internal->thread_ctx;
 
-    if (f->owner[field]->debug&FF_DEBUG_THREADS)
-        av_log(f->owner[field], AV_LOG_DEBUG,
-               "%p finished %d field %d\n", progress, n, field);
-
     pthread_mutex_lock(&p->progress_mutex);
+    debug_mv = f->owner[field]->debug&FF_DEBUG_THREADS;
 
     atomic_store_explicit(&progress[field], n, memory_order_release);
 
     pthread_cond_broadcast(&p->progress_cond);
     pthread_mutex_unlock(&p->progress_mutex);
+
+    if (debug_mv)
+        av_log(f->owner[field], AV_LOG_DEBUG,
+               "%p finished %d field %d\n", progress, n, field);
 }
 
 void ff_thread_await_progress(ThreadFrame *f, int n, int field)
 {
     PerThreadContext *p;
     atomic_int *progress = f->progress ? (atomic_int*)f->progress->data : NULL;
+    int debug_mv;
 
     if (!progress ||
         atomic_load_explicit(&progress[field], memory_order_acquire) >= n)
@@ -589,14 +592,15 @@ void ff_thread_await_progress(ThreadFrame *f, int n, int field)
 
     p = f->owner[field]->internal->thread_ctx;
 
-    if (f->owner[field]->debug&FF_DEBUG_THREADS)
-        av_log(f->owner[field], AV_LOG_DEBUG,
-               "thread awaiting %d field %d from %p\n", n, field, progress);
-
     pthread_mutex_lock(&p->progress_mutex);
+    debug_mv = f->owner[field]->debug&FF_DEBUG_THREADS;
     while (atomic_load_explicit(&progress[field], memory_order_relaxed) < n)
         pthread_cond_wait(&p->progress_cond, &p->progress_mutex);
     pthread_mutex_unlock(&p->progress_mutex);
+
+    if (debug_mv)
+        av_log(f->owner[field], AV_LOG_DEBUG,
+               "thread awaited %d field %d from %p\n", n, field, progress);
 }
 
 void ff_thread_finish_setup(AVCodecContext *avctx) {
-- 
2.8.1