[Mesa-dev] [PATCH v5 1/4] dri: Extend __DRIbackgroundCallableExtensionRec to include a callback that checks for thread safety

[Gregory Hainaut]

DRI-drivers could call Xlib functions, for example to allocate a new back
buffer.

When glthread is enabled, the driver runs mostly on a separate thread.
Therefore we need to guarantee the thread safety between libX11 calls
from the applications (not aware of the extra thread) and the ones from
the driver.

See discussion thread:
   https://lists.freedesktop.org/archives/mesa-dev/2017-April/152547.html

Fortunately, Xlib allows to lock display to ensure thread safety but
XInitThreads must be called first by the application to initialize the lock
function pointer. This patch will allow to check XInitThreads was called
to allow glthread on GLX or EGL platform.

Note: a tentative was done to port libX11 code to XCB but it didn't solve fully
thread safety.
See discussion thread:
   https://lists.freedesktop.org/archives/mesa-dev/2017-April/153137.html

Note: Nvidia forces the driver to call XInitThreads. Quoting their manpage:
"The NVIDIA OpenGL driver will automatically attempt to enable Xlib
thread-safe mode if needed. However, it might not be possible in some
situations, such as when the NVIDIA OpenGL driver library is dynamically
loaded after Xlib has been loaded and initialized. If that is the case,
threaded optimizations will stay disabled unless the application is
modified to call XInitThreads() before initializing Xlib or to link
directly against the NVIDIA OpenGL driver library. Alternatively, using
the LD_PRELOAD environment variable to include the NVIDIA OpenGL driver
library should also achieve the desired result."

v2: based on Nicolai and Matt feedback
Use C style comment

v3: based on Emil feedback
split the patch in 3
s/isGlThreadSafe/isThreadSafe/

v5: based on Marek comment
Add a comment that isThreadSafe is supported by extension v2

Signed-off-by: Gregory Hainaut <gregory.hainaut at gmail.com>
---
 include/GL/internal/dri_interface.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/GL/internal/dri_interface.h b/include/GL/internal/dri_interface.h
index c83056aa70..ffe99499fc 100644
--- a/include/GL/internal/dri_interface.h
+++ b/include/GL/internal/dri_interface.h
@@ -1714,13 +1714,26 @@ struct __DRIbackgroundCallableExtensionRec {
     * non-background thread (i.e. a thread that has already been bound to a
     * context using __DRIcoreExtensionRec::bindContext()); when this happens,
     * the \c loaderPrivate pointer must be equal to the pointer that was
     * passed to the driver when the currently bound context was created.
     *
     * This call should execute quickly enough that the driver can call it with
     * impunity whenever a background thread starts performing drawing
     * operations (e.g. it should just set a thread-local variable).
     */
    void (*setBackgroundContext)(void *loaderPrivate);
+
+   /**
+    * Indicate that it is multithread safe to use glthread.  For GLX/EGL
+    * platforms using Xlib, that involves calling XInitThreads, before
+    * opening an X display.
+    *
+    * Note: only supported if extension version is at least 2.
+    *
+    * \param loaderPrivate is the value that was passed to to the driver when
+    * the context was created.  This can be used by the loader to identify
+    * which context any callbacks are associated with.
+    */
+   GLboolean (*isThreadSafe)(void *loaderPrivate);
 };
 
 #endif
-- 
2.11.0