[vlc-commits] [Git][videolan/vlc][master] preparser: fix/simplify synchronisation

[Rémi Denis-Courmont (@Courmisch)]

Rémi Denis-Courmont pushed to branch master at VideoLAN / VLC


Commits:
581a2fe9 by Rémi Denis-Courmont at 2021-10-10T17:45:02+00:00
preparser: fix/simplify synchronisation

* Do not share the notification object for two different events.
* Use binary semaphores instead of boolean and condition variables.
* Use an relaxed atomic variable, get rid of unnecessary lock.

In normal cases, the preparse status would not need to be an atomic
variable, as the preparse semaphore serves as proper barrier. However
in the abort case, the status is written by the "wrong" thread.
Still, we can rely on the semaphore to provide a synchronisation
barrier on the status value.

- - - - -


1 changed file:

- src/preparser/preparser.c


Changes:

=====================================
src/preparser/preparser.c
=====================================
@@ -55,12 +55,9 @@ struct task
 
     input_item_parser_id_t *parser;
 
-    vlc_mutex_t lock;
-    vlc_cond_t cond_ended;
-    bool preparse_ended;
-    int preparse_status;
-    bool fetch_ended;
-
+    vlc_sem_t preparse_ended;
+    vlc_sem_t fetch_ended;
+    atomic_int preparse_status;
     atomic_bool interrupted;
 
     struct vlc_runnable runnable; /**< to be passed to the executor */
@@ -93,12 +90,9 @@ TaskNew(input_preparser_t *preparser, input_item_t *item,
     input_item_Hold(item);
 
     task->parser = NULL;
-    vlc_mutex_init(&task->lock);
-    vlc_cond_init(&task->cond_ended);
-    task->preparse_ended = false;
-    task->preparse_status = ITEM_PREPARSE_SKIPPED;
-    task->fetch_ended = false;
-
+    vlc_sem_init(&task->preparse_ended, 0);
+    vlc_sem_init(&task->fetch_ended, 0);
+    atomic_init(&task->preparse_status, ITEM_PREPARSE_SKIPPED);
     atomic_init(&task->interrupted, false);
 
     task->runnable.run = RunnableRun;
@@ -133,9 +127,11 @@ PreparserRemoveTask(input_preparser_t *preparser, struct task *task)
 static void
 NotifyPreparseEnded(struct task *task)
 {
-    if (task->cbs && task->cbs->on_preparse_ended)
-        task->cbs->on_preparse_ended(task->item, task->preparse_status,
-                                     task->userdata);
+    if (task->cbs && task->cbs->on_preparse_ended) {
+        int status = atomic_load_explicit(&task->preparse_status,
+                                          memory_order_relaxed);
+        task->cbs->on_preparse_ended(task->item, status, task->userdata);
+    }
 }
 
 static void
@@ -151,14 +147,11 @@ OnParserEnded(input_item_t *item, int status, void *task_)
          */
         return;
 
-    vlc_mutex_lock(&task->lock);
-    assert(!task->preparse_ended);
-    task->preparse_status = status == VLC_SUCCESS ? ITEM_PREPARSE_DONE
-                                                  : ITEM_PREPARSE_FAILED;
-    task->preparse_ended = true;
-    vlc_mutex_unlock(&task->lock);
-
-    vlc_cond_signal(&task->cond_ended);
+    atomic_store_explicit(&task->preparse_status,
+                          status == VLC_SUCCESS ? ITEM_PREPARSE_DONE
+                                                : ITEM_PREPARSE_FAILED,
+                          memory_order_relaxed);
+    vlc_sem_post(&task->preparse_ended);
 }
 
 static void
@@ -180,12 +173,7 @@ OnArtFetchEnded(input_item_t *item, bool fetched, void *userdata)
 
     struct task *task = userdata;
 
-    vlc_mutex_lock(&task->lock);
-    assert(!task->fetch_ended);
-    task->fetch_ended = true;
-    vlc_mutex_unlock(&task->lock);
-
-    vlc_cond_signal(&task->cond_ended);
+    vlc_sem_post(&task->fetch_ended);
 }
 
 static const input_fetcher_callbacks_t input_fetcher_callbacks = {
@@ -204,31 +192,21 @@ Parse(struct task *task, vlc_tick_t deadline)
     task->parser = input_item_Parse(task->item, obj, &cbs, task);
     if (!task->parser)
     {
-        task->preparse_status = ITEM_PREPARSE_FAILED;
+        atomic_store_explicit(&task->preparse_status, ITEM_PREPARSE_FAILED,
+                              memory_order_relaxed);
         return;
     }
 
     /* Wait until the end of parsing */
-    vlc_mutex_lock(&task->lock);
     if (deadline == VLC_TICK_INVALID)
-    {
-        while (!task->preparse_ended)
-            vlc_cond_wait(&task->cond_ended, &task->lock);
-    }
+        vlc_sem_wait(&task->preparse_ended);
     else
-    {
-        bool timeout = false;
-        while (!task->preparse_ended && !timeout)
-            timeout =
-                vlc_cond_timedwait(&task->cond_ended, &task->lock, deadline);
-
-        if (timeout)
+        if (vlc_sem_timedwait(&task->preparse_ended, deadline))
         {
-            task->preparse_status = ITEM_PREPARSE_TIMEOUT;
+            atomic_store_explicit(&task->preparse_status,
+                                  ITEM_PREPARSE_TIMEOUT, memory_order_relaxed);
             atomic_store(&task->interrupted, true);
         }
-    }
-    vlc_mutex_unlock(&task->lock);
 
     /* This call also interrupts the parsing if it is still running */
     input_item_parser_id_Release(task->parser);
@@ -249,10 +227,7 @@ Fetch(struct task *task)
         return;
 
     /* Wait until the end of fetching (fetching is not interruptible) */
-    vlc_mutex_lock(&task->lock);
-    while (!task->fetch_ended)
-        vlc_cond_wait(&task->cond_ended, &task->lock);
-    vlc_mutex_unlock(&task->lock);
+    vlc_sem_wait(&task->fetch_ended);
 }
 
 static void
@@ -291,11 +266,9 @@ Interrupt(struct task *task)
     atomic_store(&task->interrupted, true);
 
     /* Wake up the preparser cond_wait */
-    vlc_mutex_lock(&task->lock);
-    task->preparse_status = ITEM_PREPARSE_TIMEOUT;
-    task->preparse_ended = true;
-    vlc_mutex_unlock(&task->lock);
-    vlc_cond_signal(&task->cond_ended);
+    atomic_store_explicit(&task->preparse_status, ITEM_PREPARSE_TIMEOUT,
+                          memory_order_relaxed);
+    vlc_sem_post(&task->preparse_ended);
 }
 
 input_preparser_t* input_preparser_New( vlc_object_t *parent )



View it on GitLab: https://code.videolan.org/videolan/vlc/-/commit/581a2fe94681a24444fcad40ea9fe37b533429c8

-- 
View it on GitLab: https://code.videolan.org/videolan/vlc/-/commit/581a2fe94681a24444fcad40ea9fe37b533429c8
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