Hang when using glWaitSync with multithreaded shared GL contexts
Submitted by Emmanuel Durand
Assigned to Default DRI bug account
Created attachment 145472 Output of dmesg
I develop a tool which uses a separate thread for uploading textures to the GPU, in parallel to the rendering thread. These two threads are synchronized using OpenGL fences, which prevents the rendering to happen while a texture is being copied from a PBO.
On recent AMD hardware (tested on a Vega 56 and a Radeon VII) this setup hangs almost instantaneously. From my tests it seems that it waits for a glWaitSync to finish. The exact same code runs flawlessly on Intel (Mesa driver) and Nvidia (proprietary driver).
I managed to somewhat reproduce the issue in a simpler code, which merely creates two shared OpenGL contexts and does nothing except creating fences and waiting for the other thread. This example hangs with AMDGPU driver, but once again runs fine on Intel (Mesa driver) and Nvidia (proprietary driver).
I'll attach the code to this thread, and it can be found here too: https://gitlab.com/sat-metalab/splash/blob/fix/radeon_test/tests/sandbox/radeon_mesa_shared_context_freeze.cpp.
Attachment 145472, "Output of dmesg":