[FFmpeg-cvslog] avutil/cpu: Fix race condition in av_cpu_count()

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at gmail.com> | Wed Dec  2 23:57:16 2020 +0100| [a2a38b160620d91bc3f895dadc4501c589998b9c] | committer: Andreas Rheinhardt

avutil/cpu: Fix race condition in av_cpu_count()

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>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=a2a38b160620d91bc3f895dadc4501c589998b9c
---

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

diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 52f6b9a3bf..8e3576a1f3 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 = ATOMIC_VAR_INIT(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;
 }