egl: Revisit global data locking.

Lock the global mutex in _eglPreloadDrivers and _eglAllocScreenHandle.
Add comments to why certain pathes do not need locking.

src/egl/main/egldriver.c

@@ -237,6 +237,10 @@ _eglMatchDriver(_EGLDisplay *dpy)
    _EGLDriver *best_drv = NULL;
    EGLint best_score = -1, i;
 
+   /*
+    * this function is called after preloading and the drivers never change
+    * after preloading.
+    */
    for (i = 0; i < _eglGlobal.NumDrivers; i++) {
       _EGLDriver *drv = _eglGlobal.Drivers[i];
       EGLint score;
@@ -529,14 +533,21 @@ _eglPreloadDrivers(void)
 {
    EGLBoolean loaded;
 
+   /* protect the preloading process */
+   _eglLockMutex(_eglGlobal.Mutex);
+
    /* already preloaded */
-   if (_eglGlobal.NumDrivers)
+   if (_eglGlobal.NumDrivers) {
+      _eglUnlockMutex(_eglGlobal.Mutex);
       return EGL_TRUE;
+   }
 
    loaded = (_eglPreloadUserDriver() ||
              _eglPreloadDisplayDrivers() ||
              _eglPreloadDefaultDriver());
 
+   _eglUnlockMutex(_eglGlobal.Mutex);
+
    return loaded;
 }
 
@@ -548,6 +559,8 @@ void
 _eglUnloadDrivers(void)
 {
    EGLint i;
+
+   /* this is called at atexit time */
    for (i = 0; i < _eglGlobal.NumDrivers; i++) {
       _EGLDriver *drv = _eglGlobal.Drivers[i];
       lib_handle handle = drv->LibHandle;

src/egl/main/eglglobals.h

@@ -18,6 +18,7 @@ struct _egl_global
 
    EGLScreenMESA FreeScreenHandle;
 
+   /* these never change after preloading */
    EGLint NumDrivers;
    _EGLDriver *Drivers[10];
 

src/egl/main/eglscreen.c

@@ -22,17 +22,22 @@
 #include "eglconfig.h"
 #include "eglsurface.h"
 #include "eglscreen.h"
+#include "eglmutex.h"
 
 
 /**
  * Return a new screen handle/ID.
  * NOTE: we never reuse these!
  */
-EGLScreenMESA
+static EGLScreenMESA
 _eglAllocScreenHandle(void)
 {
-   EGLScreenMESA s = _eglGlobal.FreeScreenHandle;
-   _eglGlobal.FreeScreenHandle++;
+   EGLScreenMESA s;
+   
+   _eglLockMutex(_eglGlobal.Mutex);
+   s = _eglGlobal.FreeScreenHandle++;
+   _eglUnlockMutex(_eglGlobal.Mutex);
+
    return s;
 }
 

src/egl/main/eglscreen.h

@@ -29,10 +29,6 @@ struct _egl_screen
 };
 
 
-extern EGLScreenMESA
-_eglAllocScreenHandle(void);
-
-
 PUBLIC void
 _eglInitScreen(_EGLScreen *screen);