[Mesa-dev] [PATCH mesa 2/2] egl/display: make platform detection thread-safe

[Eric Engestrom]

If the detections methods ever become able to return different results
for different threads, the if chain might make threads go back and forth
between invalid and valid platforms.
Solve this by doing the detection in a local var and only overwriting
the global one at the end, if no other thread has updated it since.

This means the platform detected in the thread might not be the platform
returned by the function, but this is a different issue that will need
to be discussed when this becomes possible.

Reported-by: Grazvydas Ignotas <notasas at gmail.com>
Signed-off-by: Eric Engestrom <eric.engestrom at imgtec.com>
---

This is unnecessary in my opinion, but doesn't hurt :)
Putting it out there as a possible fix, but my vote is to not merge it,
not until this per-thread-platform-detection becomes possible anyway.
---
 src/egl/main/egldisplay.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/src/egl/main/egldisplay.c b/src/egl/main/egldisplay.c
index 3f7752f53d..92586def0c 100644
--- a/src/egl/main/egldisplay.c
+++ b/src/egl/main/egldisplay.c
@@ -36,6 +36,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include "c11/threads.h"
+#include "util/u_atomic.h"
 
 #include "eglcontext.h"
 #include "eglcurrent.h"
@@ -181,23 +182,27 @@ _EGLPlatformType
 _eglGetNativePlatform(void *nativeDisplay)
 {
    static _EGLPlatformType native_platform = _EGL_INVALID_PLATFORM;
+   _EGLPlatformType detected_platform = native_platform;
    char *detection_method = NULL;
 
-   if (native_platform == _EGL_INVALID_PLATFORM) {
-      native_platform = _eglGetNativePlatformFromEnv();
+   if (detected_platform == _EGL_INVALID_PLATFORM) {
+      detected_platform = _eglGetNativePlatformFromEnv();
       detection_method = "environment overwrite";
    }
 
-   if (native_platform == _EGL_INVALID_PLATFORM) {
-      native_platform = _eglNativePlatformDetectNativeDisplay(nativeDisplay);
+   if (detected_platform == _EGL_INVALID_PLATFORM) {
+      detected_platform = _eglNativePlatformDetectNativeDisplay(nativeDisplay);
       detection_method = "autodetected";
    }
 
-   if (native_platform == _EGL_INVALID_PLATFORM) {
-      native_platform = _EGL_NATIVE_PLATFORM;
+   if (detected_platform == _EGL_INVALID_PLATFORM) {
+      detected_platform = _EGL_NATIVE_PLATFORM;
       detection_method = "build-time configuration";
    }
 
+   p_atomic_cmpxchg(&native_platform, _EGL_INVALID_PLATFORM,
+                    detected_platform);
+
    if (detection_method != NULL) {
       _eglLog(_EGL_DEBUG, "Native platform type: %s (%s)",
               egl_platforms[native_platform].name, detection_method);
-- 
Cheers,
  Eric