avutil: fix data race in av_get_cpu_flags()

Make the one-time initialization in av_get_cpu_flags() thread-safe. The
static variable |cpu_flags| in libavutil/cpu.c is read and written using
normal load and store operations. These are considered as data races.
The fix is to use atomic load and store operations.

The fix can be verified by running the libavutil/tests/cpu_init.c test
program under ThreadSanitizer:
    ./configure --toolchain=clang-tsan
    make libavutil/tests/cpu_init
    libavutil/tests/cpu_init

There should be no warnings from ThreadSanitizer.

Co-author: Dmitry Vyukov of Google, who suggested the data race fix.
Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/cpu.c

@@ -17,6 +17,7 @@
  */
 
 #include <stdint.h>
+#include <stdatomic.h>
 
 #include "cpu.h"
 #include "cpu_internal.h"
@@ -44,7 +45,7 @@
 #include <unistd.h>
 #endif
 
-static int cpu_flags = -1;
+static atomic_int cpu_flags = ATOMIC_VAR_INIT(-1);
 
 static int get_cpu_flags(void)
 {
@@ -82,22 +83,23 @@ void av_force_cpu_flags(int arg){
         arg |= AV_CPU_FLAG_MMX;
     }
 
-    cpu_flags = arg;
+    atomic_store_explicit(&cpu_flags, arg, memory_order_relaxed);
 }
 
 int av_get_cpu_flags(void)
 {
-    int flags = cpu_flags;
+    int flags = atomic_load_explicit(&cpu_flags, memory_order_relaxed);
     if (flags == -1) {
         flags = get_cpu_flags();
-        cpu_flags = flags;
+        atomic_store_explicit(&cpu_flags, flags, memory_order_relaxed);
     }
     return flags;
 }
 
 void av_set_cpu_flags_mask(int mask)
 {
-    cpu_flags = get_cpu_flags() & mask;
+    atomic_store_explicit(&cpu_flags, get_cpu_flags() & mask,
+                          memory_order_relaxed);
 }
 
 int av_parse_cpu_flags(const char *s)

libavutil/cpu.h

@@ -85,8 +85,6 @@ void av_force_cpu_flags(int flags);
  * Set a mask on flags returned by av_get_cpu_flags().
  * This function is mainly useful for testing.
  * Please use av_force_cpu_flags() and av_get_cpu_flags() instead which are more flexible
- *
- * @warning this function is not thread safe.
  */
 attribute_deprecated void av_set_cpu_flags_mask(int mask);