Making torch/csrc/cuda nccl usage safe for nccl 2.5 #29014
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ezyang
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Nov 1, 2019
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@mcarilli Thanks for the fix! The code here is used by single-process multi-GPU code. |
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Thanks to @AddyLaddy @ptrblck for tracking this fix down.
In torch/csrc/cuda/nccl.cpp and torch/csrc/cuda/python_nccl.cpp, construction of the
AutoNcclGroupguard (which callsncclGroupStart()) precedes a possible call toget_communicators, which may callncclCommInitAll(). CallingncclCommInitAll()within ancclGroupStart()/End()is incorrect according to our Nccl people.It seemed ok (relevant tests were silently passing) as long as Pytorch was compiled/linked against Nccl 2.4.x (which is currently what's locked into your third_party/nccl subrepo). However, when we tried to compile and link against Nccl 2.5.x in internal builds, we began to see test hangs (TestAutogradDeviceTypeCUDA.test_unused_output_device_cuda was what initially brought it to our attention).
The present PR fixes those hangs, as far as we know, and will prevent a nasty future surprise when you start building against nccl 2.5.
The backend affected by this PR is exposed via https://github.com/pytorch/pytorch/blob/master/torch/cuda/nccl.py. I'm not sure if the exposure is actually used anywhere (I think the distributed frontend is now backed by ProcessGroupNCCL in torch/lib/c10d). So this PR may affect code that is already dead or dying, but still tested, it seems.
I skimmed ProcessGroupNCCL.cpp for potential similar vulnerabilities and didn't spot anything obvious.