avutil: fix data race in av_get_cpu_flags()

Make the one-time initialization in av_get_cpu_flags() thread-safe. The
static variables |flags|, |cpuflags_mask|, and |checked| in
libavutil/cpu.c are 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.

Remove the |checked| variable because the invalid value of -1 for
|flags| can be used to indicate the same condition. Rename |flags| to
|cpu_flags| and move it to file scope.

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>

1 files changed, 18 insertions, 16 deletions

diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 7d7390a756..0109c9e8d1 100644
--- a/libavutil/cpu.c
+++ b/libavutil/cpu.c
@@ -17,6 +17,7 @@
  */
 
 #include <stdint.h>
+#include <stdatomic.h>
 
 #include "cpu.h"
 #include "cpu_internal.h"
@@ -42,34 +43,35 @@
 #include <unistd.h>
 #endif
 
-static int cpuflags_mask = -1, checked;
+static atomic_int cpu_flags = ATOMIC_VAR_INIT(-1);
 
-int av_get_cpu_flags(void)
+static int get_cpu_flags(void)
 {
-    static int flags;
-
-    if (checked)
-        return flags;
-
     if (ARCH_AARCH64)
-        flags = ff_get_cpu_flags_aarch64();
+        return ff_get_cpu_flags_aarch64();
     if (ARCH_ARM)
-        flags = ff_get_cpu_flags_arm();
+        return ff_get_cpu_flags_arm();
     if (ARCH_PPC)
-        flags = ff_get_cpu_flags_ppc();
+        return ff_get_cpu_flags_ppc();
     if (ARCH_X86)
-        flags = ff_get_cpu_flags_x86();
-
-    flags  &= cpuflags_mask;
-    checked = 1;
+        return ff_get_cpu_flags_x86();
+    return 0;
+}
 
+int av_get_cpu_flags(void)
+{
+    int flags = atomic_load_explicit(&cpu_flags, memory_order_relaxed);
+    if (flags == -1) {
+        flags = get_cpu_flags();
+        atomic_store_explicit(&cpu_flags, flags, memory_order_relaxed);
+    }
     return flags;
 }
 
 void av_set_cpu_flags_mask(int mask)
 {
-    cpuflags_mask = mask;
-    checked       = 0;
+    atomic_store_explicit(&cpu_flags, get_cpu_flags() & mask,
+                          memory_order_relaxed);
 }
 
 int av_parse_cpu_flags(const char *s)