Fix deadlock for multi-output forward AD #67995
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lezcano
approved these changes
Nov 9, 2021
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Will hide some of the issues from #67367
This will at least allow us to run gradcheck for now until the above issue is fixed.
For more context, the deadlock happens when we (wrongfully) set a forward grad that also has a forward grad of the same level.
In particular, when exiting the level from
pytorch/torch/csrc/autograd/forward_grad.cpp
Line 23 in 191b48b
We are taking the
all_forward_levels_mutex_lock and proceed to delete the level atpytorch/torch/csrc/autograd/forward_grad.cpp
Line 29 in 191b48b
In the level destructor in
pytorch/torch/csrc/autograd/forward_grad.cpp
Line 55 in 191b48b
But in the (bad) case where this Tensor also has a forward grad for this level, the autograd meta clears the fw grads:
pytorch/torch/csrc/autograd/forward_grad.h
Line 124 in 191b48b
While clearing, we access the level (to de-register this forward grad) via
pytorch/torch/csrc/autograd/forward_grad.h
Line 139 in 191b48b
But this tries to access the level again in
pytorch/torch/csrc/autograd/forward_grad.cpp
Line 39 in 191b48b