Fix python gc race condition with THPVariable_traverse #4437
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apaszke
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Jan 2, 2018
apaszke
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Jan 2, 2018
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Fixes #3883: asserts being triggered in DataParallel for long training regiments like imagenet.
The bug is that a python Function in the computation graph gets garbage collected due to a C++ shared pointer to the C++ Function it wraps changing
use_count()from 1 to more than 1 during python garbage collection. This happens because the "python state" of Variable changes: theTHPVariable_traversemethod indicates that when the shared pointer'suse_count()is 1, the python Variable holds a python reference to the python Function, but not whenuse_count() > 1. See this gist for a more detailed exploration of the diagnosis.The bug could be fixed by treating the shared pointer (a Variable's
grad_fn) as part of the python state and intelligently grabbing the GIL when necessary. The downside to this approach is the complexity and potential overhead.Alternatively, after talking with @colesbury, we think it might be better to remove the checking of the shared pointer's
use_count()inTHPVariable_traverse. This effectively disables reference cycle garbage collection for python Functions.