[FFmpeg-devel] [PATCH 221/223] avutil/cpu: Fix race condition in av_cpu_count()

[Andreas Rheinhardt]

av_cpu_count() intends to emit a debug message containing the number of
logical cores when called the first time. The check currently works with
a static volatile int; yet this does not help at all in case of
concurrent accesses by multiple threads. So replace this with an
atomic_int.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
To test my patches I have modified avcodec_open2() to always open the
desired AVCodecContext simultaneously from two different threads [1].
With this one easily runs into this issue when one uses a decoder with
FF_CODEC_CAP_INIT_THREADSAFE that supports threading.

[1]: https://pastebin.com/a1uEuQ81

 libavutil/cpu.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 52f6b9a3bf..7b8a9045ab 100644
--- a/libavutil/cpu.c
+++ b/libavutil/cpu.c
@@ -274,7 +274,7 @@ int av_parse_cpu_caps(unsigned *flags, const char *s)
 
 int av_cpu_count(void)
 {
-    static volatile int printed;
+    static atomic_int printed = 0;
 
     int nb_cpus = 1;
 #if HAVE_WINRT
@@ -306,10 +306,8 @@ int av_cpu_count(void)
     nb_cpus = sysinfo.dwNumberOfProcessors;
 #endif
 
-    if (!printed) {
+    if (!atomic_exchange_explicit(&printed, 1, memory_order_relaxed))
         av_log(NULL, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);
-        printed = 1;
-    }
 
     return nb_cpus;
 }
-- 
2.25.1