[FFmpeg-devel] [PATCH 3/3] avutil/buffer: Don't use atomics for AVBufferPool.refcount

[Andreas Rheinhardt]

The AVBufferPool is already guarded by a mutex, so one can
just move use said mutex to also protect modifying the refcount.
Doing so yields a minor decrease in complexity.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
Currently freeing the entries is serialized; the API documentation
does not force us to do so ("is thread-safe as long as either the
default alloc callback is used, or the user-supplied one is
thread-safe"). I could easily implement unserializing it if desired,
yet it would be incompatible with this patch.

 libavutil/buffer.c          | 17 ++++++++++-------
 libavutil/buffer_internal.h |  2 +-
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/libavutil/buffer.c b/libavutil/buffer.c
index e4562a79b1..2335a99e9f 100644
--- a/libavutil/buffer.c
+++ b/libavutil/buffer.c
@@ -272,7 +272,7 @@ AVBufferPool *av_buffer_pool_init2(size_t size, void *opaque,
     pool->alloc     = av_buffer_alloc; // fallback
     pool->pool_free = pool_free;
 
-    atomic_init(&pool->refcount, 1);
+    pool->refcount = 1;
 
     return pool;
 }
@@ -288,7 +288,7 @@ AVBufferPool *av_buffer_pool_init(size_t size, AVBufferRef* (*alloc)(size_t size
     pool->size     = size;
     pool->alloc    = alloc ? alloc : av_buffer_alloc;
 
-    atomic_init(&pool->refcount, 1);
+    pool->refcount = 1;
 
     return pool;
 }
@@ -322,6 +322,7 @@ static void buffer_pool_free(AVBufferPool *pool)
 void av_buffer_pool_uninit(AVBufferPool **ppool)
 {
     AVBufferPool *pool;
+    unsigned refcount;
 
     if (!ppool || !*ppool)
         return;
@@ -330,9 +331,10 @@ void av_buffer_pool_uninit(AVBufferPool **ppool)
 
     ff_mutex_lock(&pool->mutex);
     buffer_pool_flush(pool);
+    refcount = --pool->refcount;
     ff_mutex_unlock(&pool->mutex);
 
-    if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
+    if (!refcount)
         buffer_pool_free(pool);
 }
 
@@ -340,13 +342,15 @@ static void pool_release_buffer(void *opaque, uint8_t *data)
 {
     BufferPoolEntry *buf = opaque;
     AVBufferPool *pool = buf->pool;
+    unsigned refcount;
 
     ff_mutex_lock(&pool->mutex);
     buf->next = pool->pool;
     pool->pool = buf;
+    refcount   = --pool->refcount;
     ff_mutex_unlock(&pool->mutex);
 
-    if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
+    if (!refcount)
         buffer_pool_free(pool);
 }
 
@@ -400,10 +404,9 @@ AVBufferRef *av_buffer_pool_get(AVBufferPool *pool)
     } else {
         ret = pool_alloc_buffer(pool);
     }
-    ff_mutex_unlock(&pool->mutex);
-
     if (ret)
-        atomic_fetch_add_explicit(&pool->refcount, 1, memory_order_relaxed);
+        pool->refcount++;
+    ff_mutex_unlock(&pool->mutex);
 
     return ret;
 }
diff --git a/libavutil/buffer_internal.h b/libavutil/buffer_internal.h
index bdff1b5b32..a726e5e865 100644
--- a/libavutil/buffer_internal.h
+++ b/libavutil/buffer_internal.h
@@ -99,7 +99,7 @@ struct AVBufferPool {
      * buffers have been released, then it's safe to free the pool and all
      * the buffers in it.
      */
-    atomic_uint refcount;
+    unsigned refcount;
 
     size_t size;
     void *opaque;
-- 
2.30.2