data race in GC alloc counters

[vchuravy]

Running under TSAN with JULIA_NUM_THREADS=4 gives the following warning:

WARNING: ThreadSanitizer: data race (pid=32587)
  Write of size 8 at 0x7f82ec049d80 by thread T3:
    #0 jl_gc_pool_alloc /home/vchuravy/src/julia/src/gc.c:959:9 (libjulia.so.0.7+0xc2da4)
    #1 jl_gc_alloc_ /home/vchuravy/src/julia/src/./julia_internal.h:274:13 (libjulia.so.0.7+0x7a031)
    #2 jl_init_root_task /home/vchuravy/src/julia/src/task.c:714 (libjulia.so.0.7+0x7a031)
    #3 ti_threadfun /home/vchuravy/src/julia/src/threading.c:367:5 (libjulia.so.0.7+0xbe187)

  Previous write of size 8 at 0x7f82ec049d80 by thread T2:
    #0 jl_gc_pool_alloc /home/vchuravy/src/julia/src/gc.c:959:9 (libjulia.so.0.7+0xc2da4)
    #1 jl_gc_alloc_ /home/vchuravy/src/julia/src/./julia_internal.h:274:13 (libjulia.so.0.7+0x7a031)
    #2 jl_init_root_task /home/vchuravy/src/julia/src/task.c:714 (libjulia.so.0.7+0x7a031)
    #3 ti_threadfun /home/vchuravy/src/julia/src/threading.c:367:5 (libjulia.so.0.7+0xbe187)

  Location is global 'gc_num' of size 104 at 0x7f82ec049d80 (libjulia.so.0.7+0x00000057fd80)

  Thread T3 (tid=32591, running) created by main thread at:
    #0 pthread_create /home/vchuravy/src/julia/deps/srccache/llvm-6.0.0/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:992:3 (julia+0x40f766)
    #1 uv_thread_create /home/vchuravy/src/julia/deps/srccache/libuv-be317349252699670131395f125c3861d793ca86/src/unix/thread.c:76:9 (libjulia.so.0.7+0x247f22)
    #2 _julia_init /home/vchuravy/src/julia/src/init.c:726:5 (libjulia.so.0.7+0x768dc)
    #3 julia_init__threading /home/vchuravy/src/julia/src/task.c:302:5 (libjulia.so.0.7+0x79420)
    #4 main /home/vchuravy/src/julia/ui/repl.c:234:5 (julia+0x4994d3)

  Thread T2 (tid=32590, running) created by main thread at:
    #0 pthread_create /home/vchuravy/src/julia/deps/srccache/llvm-6.0.0/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:992:3 (julia+0x40f766)
    #1 uv_thread_create /home/vchuravy/src/julia/deps/srccache/libuv-be317349252699670131395f125c3861d793ca86/src/unix/thread.c:76:9 (libjulia.so.0.7+0x247f22)
    #2 _julia_init /home/vchuravy/src/julia/src/init.c:726:5 (libjulia.so.0.7+0x768dc)
    #3 julia_init__threading /home/vchuravy/src/julia/src/task.c:302:5 (libjulia.so.0.7+0x79420)
    #4 main /home/vchuravy/src/julia/ui/repl.c:234:5 (julia+0x4994d3)

SUMMARY: ThreadSanitizer: data race /home/vchuravy/src/julia/src/gc.c:959:9 in jl_gc_pool_alloc

--edit: Already has a // FIXME - need JL_ATOMIC_FETCH_AND_ADD here annotation

[yuyichao]

This is real though it shouldn't matter for most purposes ATM. The FIXME note is wrong though. Simply doing it will be way too slow.