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Created attachment 2709 [details]
patch

SDL_AtomicGet and SDL_AtomicGetPtr attempt to perform an atomic load by repeatedly doing non-atomic loads and trying a compare and swap the loaded value with itself. The non-atomic loads are flagged as errors by thread sanitizer. For example:
WARNING: ThreadSanitizer: data race (pid=8625)
  Read of size 4 at 0x7fb162d62240 by main thread:
    #0 SDL_AtomicGet_REAL SDL2-2.0.5/src/atomic/SDL_atomic.c:216
    #1 SDL_AddTimer_REAL SDL2-2.0.5/src/timer/SDL_timer.c:285
    #2 SDL_AddTimer SDL2-2.0.5/src/dynapi/SDL_dynapi_procs.h:520

  Previous atomic write of size 4 at 0x7fb162d62240 by thread T1:
    #0 __tsan_atomic32_compare_exchange_weak <null> (libtsan.so.0+0x000000064163)
    #1 SDL_AtomicCAS_REAL SDL2-2.0.5/src/atomic/SDL_atomic.c:94
    #2 SDL_AtomicGet_REAL SDL2-2.0.5/src/atomic/SDL_atomic.c:217
    #3 SDL_TimerThread SDL2-2.0.5/src/timer/SDL_timer.c:141
    #4 SDL_RunThread SDL2-2.0.5/src/thread/SDL_thread.c:283
    #5 RunThread SDL2-2.0.5/src/thread/pthread/SDL_systhread.c:74

 Location is global 'SDL_timer_data' of size 208 at 0x7fb162d62180

When GCC's built-in atomic operations are available, __atomic_load_n will perform an atomic load, and it will be correctly instrumented by thread sanitizer. The attached patch uses this.