[Bug 98172] Concurrent call to glClientWaitSync results in segfault in one of the waiters.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=98172

            Bug ID: 98172
           Summary: Concurrent call to glClientWaitSync results in
                    segfault in one of the waiters.
           Product: Mesa
           Version: 11.2
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/r600
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: shinji.suzuki at gmail.com
        QA Contact: dri-devel at lists.freedesktop.org

In my app, a fence is created in Thread-A and it gets passed to Thread-B and
Thread-C to be waited upon. (Each thread has its own context.)
Thread-A issues the call, fence = glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE,
0).
Thread-B and C issue the call, glClientWaitSync(fence,
GL_SYNC_FLUSH_COMMANDS_BIT, GL_TIMEOUT_IGNORED);

Most of the time, wait in both clients succeed but occasionally one of them
generates segfault. Upon inspection of generated core, it turned out so->fence
in the expression "&so->fence" at line 113 in
src/mesa/state_tracker/st_cb_syncobj.c is NULL, which should be causing the
segfault down the call chain through screen->fence_reference. I think there is
race in executing the code block. If I introduce a mutex in my app with which
to avoid concurrent call to glClientWaitSync, I don't observe segfault
happening.
Here is the snippet of code in question from st_cb_syncobj.c:

   if (so->fence &&
       screen->fence_finish(screen, so->fence, timeout)) {
      screen->fence_reference(screen, &so->fence, NULL);
      so->b.StatusFlag = GL_TRUE;
   }

My environment is;
Ubuntu 16.04 LTS
Linux a7da 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64
x86_64 x86_64 GNU/Linux
libgl1-mesa-dri:amd64 / 11.2.0-1ubuntu2.2
Radeon HD3300

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