From fed50c4304eecb352e29ce789cdb96ea84d6162f Mon Sep 17 00:00:00 2001 From: Wan-Teh Chang Date: Tue, 6 Dec 2016 18:16:13 -0800 Subject: [PATCH] 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 Signed-off-by: Michael Niedermayer --- libavutil/cpu.c | 12 +++++++----- libavutil/cpu.h | 2 -- 2 files changed, 7 insertions(+), 7 deletions(-) diff --git a/libavutil/cpu.c b/libavutil/cpu.c index 73317c4d4c6..16e0c9278fd 100644 --- a/libavutil/cpu.c +++ b/libavutil/cpu.c @@ -17,6 +17,7 @@ */ #include +#include #include "cpu.h" #include "cpu_internal.h" @@ -44,7 +45,7 @@ #include #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) diff --git a/libavutil/cpu.h b/libavutil/cpu.h index 4bff16714a3..8499f0ea8b6 100644 --- a/libavutil/cpu.h +++ b/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); -- 2.39.2