[FFmpeg-devel] [PATCH] avutil: fix data race in av_get_cpu_flags().

[Michael Niedermayer]

On Mon, Nov 21, 2016 at 03:37:49PM -0800, Wan-Teh Chang wrote:
> Make the one-time initialization in av_get_cpu_flags() thread-safe.
> The fix assumes -1 is an invalid value for cpu flags.

please explain the bug / race that occurs and how it is fixed


> 
> The fix requires new atomic functions to get, set, and compare-and-swap
> an integer without a memory barrier.

why ?


> 
> The data race fix is written by Dmitry Vyukov of Google.

Then the author for the git patch should be set accordingly


[...]

> @@ -44,7 +45,20 @@
>  #include <unistd.h>
>  #endif
>  
> -static int flags, checked;
> +static int cpu_flags = -1;
> +
> +static int get_cpu_flags(void)
> +{
> +    if (ARCH_AARCH64)
> +        return ff_get_cpu_flags_aarch64();
> +    if (ARCH_ARM)
> +        return ff_get_cpu_flags_arm();
> +    if (ARCH_PPC)
> +        return ff_get_cpu_flags_ppc();
> +    if (ARCH_X86)
> +        return ff_get_cpu_flags_x86();
> +    /* Not reached. */

src/libavutil/cpu.c: In function ‘get_cpu_flags’:
src/libavutil/cpu.c:61: error: control reaches end of non-void function

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have often repented speaking, but never of holding my tongue.
-- Xenocrates
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