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, 0 insertions, 2 deletions

diff --git a/libavutil/cpu.h b/libavutil/cpu.h
index deff4ccf55..c205ee16f6 100644
--- a/libavutil/cpu.h
+++ b/libavutil/cpu.h
@@ -73,8 +73,6 @@ int av_get_cpu_flags(void);
 /**
  * Set a mask on flags returned by av_get_cpu_flags().
  * This function is mainly useful for testing.
- *
- * @warning this function is not thread safe.
  */
 void av_set_cpu_flags_mask(int mask);