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, 2 insertions, 0 deletions

diff --git a/libavutil/Makefile b/libavutil/Makefile
index 28372c9bef..f34c79950e 100644
--- a/libavutil/Makefile
+++ b/libavutil/Makefile
@@ -145,3 +145,5 @@ TESTPROGS = adler32                                                     \
             sha                                                         \
             tree                                                        \
             xtea                                                        \
+
+TESTPROGS-$(HAVE_THREADS)               += cpu_init