Re: [FFmpeg-devel] [PATCH] Stop using _explicit atomic operations where not necessary.

James Almer Sat, 05 Jun 2021 08:08:14 -0700

On 6/5/2021 11:27 AM, Anton Khirnov wrote:

Memory ordering constraints other than the default (sequentially
consistent) can behave in very unintuitive and unexpected ways, and so
should be avoided unless there is a strong (typically performance)
reason for using them. This holds especially for memory_order_relaxed,
which imposes no ordering constraints.

Performance is important for FIFO, though. Could they use a laxer orderat least for the fetch_sub and fetch_add cases?

Also, commit fed50c4304e, which made cpu_flags an atomic type, statesthat ThreadSanitizer stopped reporting races on tests/cpu_init afterthat change, so maybe memory order is not important for it, only atomicity.In any case these cpu and mem functions are hardly a bottleneckanywhere, and rarely called outside of debugging, so this change is ok.

---
  libavformat/fifo.c |  8 ++++----
  libavutil/cpu.c    |  8 ++++----
  libavutil/mem.c    | 10 +++++-----
  3 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/libavformat/fifo.c b/libavformat/fifo.c
index 50656f78b7..2ac14207b8 100644
--- a/libavformat/fifo.c
+++ b/libavformat/fifo.c
@@ -185,7 +185,7 @@ static int fifo_thread_write_packet(FifoThreadContext *ctx, 
AVPacket *pkt)
      int ret, s_idx;

if (fifo->timeshift && pkt->dts != AV_NOPTS_VALUE)

-        atomic_fetch_sub_explicit(&fifo->queue_duration, next_duration(avf, pkt, 
&ctx->last_received_dts), memory_order_relaxed);
+        atomic_fetch_sub(&fifo->queue_duration, next_duration(avf, pkt, 
&ctx->last_received_dts));

if (ctx->drop_until_keyframe) {

          if (pkt->flags & AV_PKT_FLAG_KEY) {
@@ -454,7 +454,7 @@ static void *fifo_consumer_thread(void *data)
              av_log(avf, AV_LOG_INFO, "FIFO queue flushed\n");

if (fifo->timeshift)

-            while (atomic_load_explicit(&fifo->queue_duration, memory_order_relaxed) 
< fifo->timeshift)
+            while (atomic_load(&fifo->queue_duration) < fifo->timeshift)
                  av_usleep(10000);

ret = av_thread_message_queue_recv(queue, &msg, 0);

@@ -594,7 +594,7 @@ static int fifo_write_packet(AVFormatContext *avf, AVPacket 
*pkt)
      }

if (fifo->timeshift && pkt && pkt->dts != AV_NOPTS_VALUE)

-        atomic_fetch_add_explicit(&fifo->queue_duration, next_duration(avf, pkt, 
&fifo->last_sent_dts), memory_order_relaxed);
+        atomic_fetch_add(&fifo->queue_duration, next_duration(avf, pkt, 
&fifo->last_sent_dts));

return ret;

  fail:
@@ -621,7 +621,7 @@ static int fifo_write_trailer(AVFormatContext *avf)
              } else {
                  now += delay;
              }
-            atomic_fetch_add_explicit(&fifo->queue_duration, delay, 
memory_order_relaxed);
+            atomic_fetch_add(&fifo->queue_duration, delay);
              elapsed += delay;
              if (elapsed > fifo->timeshift)
                  break;
diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 8960415d00..8a6af81ae4 100644
--- a/libavutil/cpu.c
+++ b/libavutil/cpu.c
@@ -89,15 +89,15 @@ void av_force_cpu_flags(int arg){
          arg |= AV_CPU_FLAG_MMX;
      }

- atomic_store_explicit(&cpu_flags, arg, memory_order_relaxed);

+    atomic_store(&cpu_flags, arg);
  }

int av_get_cpu_flags(void)

  {
-    int flags = atomic_load_explicit(&cpu_flags, memory_order_relaxed);
+    int flags = atomic_load(&cpu_flags);
      if (flags == -1) {
          flags = get_cpu_flags();
-        atomic_store_explicit(&cpu_flags, flags, memory_order_relaxed);
+        atomic_store(&cpu_flags, flags);
      }
      return flags;
  }
@@ -221,7 +221,7 @@ int av_cpu_count(void)
      nb_cpus = sysinfo.dwNumberOfProcessors;
  #endif

- if (!atomic_exchange_explicit(&printed, 1, memory_order_relaxed))

+    if (!atomic_exchange(&printed, 1))
          av_log(NULL, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);

return nb_cpus;

diff --git a/libavutil/mem.c b/libavutil/mem.c
index a52d33d4a6..c216c0314f 100644
--- a/libavutil/mem.c
+++ b/libavutil/mem.c
@@ -72,14 +72,14 @@ void  free(void *ptr);
  static atomic_size_t max_alloc_size = ATOMIC_VAR_INIT(INT_MAX);

void av_max_alloc(size_t max){

-    atomic_store_explicit(&max_alloc_size, max, memory_order_relaxed);
+    atomic_store(&max_alloc_size, max);
  }

void *av_malloc(size_t size)

  {
      void *ptr = NULL;

- if (size > atomic_load_explicit(&max_alloc_size, memory_order_relaxed))

+    if (size > atomic_load(&max_alloc_size))
          return NULL;

#if HAVE_POSIX_MEMALIGN

@@ -135,7 +135,7 @@ void *av_malloc(size_t size)
  void *av_realloc(void *ptr, size_t size)
  {
      void *ret;
-    if (size > atomic_load_explicit(&max_alloc_size, memory_order_relaxed))
+    if (size > atomic_load(&max_alloc_size))
          return NULL;

#if HAVE_ALIGNED_MALLOC

@@ -489,7 +489,7 @@ void *av_fast_realloc(void *ptr, unsigned int *size, size_t 
min_size)
      if (min_size <= *size)
          return ptr;

- max_size = atomic_load_explicit(&max_alloc_size, memory_order_relaxed);

+    max_size = atomic_load(&max_alloc_size);

if (min_size > max_size) {

          *size = 0;
@@ -521,7 +521,7 @@ static inline void fast_malloc(void *ptr, unsigned int 
*size, size_t min_size, i
          return;
      }

- max_size = atomic_load_explicit(&max_alloc_size, memory_order_relaxed);

+    max_size = atomic_load(&max_alloc_size);

if (min_size > max_size) {

          av_freep(ptr);


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Re: [FFmpeg-devel] [PATCH] Stop using _explicit atomic operations where not necessary.

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