[FFmpeg-devel] [PATCH v2 2/3] avcodec/executor: remove unused ready callback

[Nuo Mi]

Due to the nature of multithreading, using a "ready check" mechanism may introduce a deadlock. For example:

Suppose all tasks have been submitted to the executor, and the last thread checks the entire list and finds
no ready tasks. It then goes to sleep, waiting for a new task. However, for some multithreading-related reason,
a task becomes ready after the check. Since no other thread is aware of this and no new tasks are being added to
the executor, a deadlock occurs.

In VVC, this function is unnecessary because we use a scoreboard. All tasks submitted to the executor are ready tasks.
---
 libavcodec/executor.c   | 6 ++----
 libavcodec/executor.h   | 3 ---
 libavcodec/vvc/thread.c | 8 --------
 3 files changed, 2 insertions(+), 15 deletions(-)

diff --git a/libavcodec/executor.c b/libavcodec/executor.c
index db80d067ac..84d52e7e3b 100644
--- a/libavcodec/executor.c
+++ b/libavcodec/executor.c
@@ -79,10 +79,8 @@ static void add_task(AVTask **prev, AVTask *t)
 static int run_one_task(AVExecutor *e, void *lc)
 {
     AVTaskCallbacks *cb = &e->cb;
-    AVTask **prev;
+    AVTask **prev = &e->tasks;
 
-    for (prev = &e->tasks; *prev && !cb->ready(*prev, cb->user_data); prev = &(*prev)->next)
-        /* nothing */;
     if (*prev) {
         AVTask *t = remove_task(prev, *prev);
         if (e->thread_count > 0)
@@ -143,7 +141,7 @@ AVExecutor* av_executor_alloc(const AVTaskCallbacks *cb, int thread_count)
 {
     AVExecutor *e;
     int has_lock = 0, has_cond = 0;
-    if (!cb || !cb->user_data || !cb->ready || !cb->run || !cb->priority_higher)
+    if (!cb || !cb->user_data || !cb->run || !cb->priority_higher)
         return NULL;
 
     e = av_mallocz(sizeof(*e));
diff --git a/libavcodec/executor.h b/libavcodec/executor.h
index 6083e41312..29fb55f66b 100644
--- a/libavcodec/executor.h
+++ b/libavcodec/executor.h
@@ -42,9 +42,6 @@ typedef struct AVTaskCallbacks {
     // return 1 if a's priority > b's priority
     int (*priority_higher)(const AVTask *a, const AVTask *b);
 
-    // task is ready for run
-    int (*ready)(const AVTask *t, void *user_data);
-
     // run the task
     int (*run)(AVTask *t, void *local_context, void *user_data);
 } AVTaskCallbacks;
diff --git a/libavcodec/vvc/thread.c b/libavcodec/vvc/thread.c
index a4d7b31c37..993b682e1b 100644
--- a/libavcodec/vvc/thread.c
+++ b/libavcodec/vvc/thread.c
@@ -372,13 +372,6 @@ static int task_is_stage_ready(VVCTask *t, int add)
     return task_has_target_score(t, stage, score);
 }
 
-static int task_ready(const AVTask *_t, void *user_data)
-{
-    VVCTask *t = (VVCTask*)_t;
-
-    return task_is_stage_ready(t, 0);
-}
-
 #define CHECK(a, b)                         \
     do {                                    \
         if ((a) != (b))                     \
@@ -689,7 +682,6 @@ AVExecutor* ff_vvc_executor_alloc(VVCContext *s, const int thread_count)
         s,
         sizeof(VVCLocalContext),
         task_priority_higher,
-        task_ready,
         task_run,
     };
     return av_executor_alloc(&callbacks, thread_count);
-- 
2.34.1