Hard freeze with `Edit as Maya` prims and `Usd.Notice` Python listeners

[jufrantz]

We are hiting maya hard freezes in production scenes.

This happens when the following are true:

• an USD prim is "edited as maya" (very common in our production scenes).
• a Python tool is listening for USD change notifications via Tf.Notice (also very common).
• a Python command - that triggers maya parallel evaluation - is executed, such as maya.cmds.mayaUSDExport / mayaUSDMergeToUsd.

Steps to reproduce

The Python reproduction below is intentionally minimal, but it still reflects the production scenario well.

from pxr import Tf, Usd
from maya import cmds
from maya.api.OpenMaya import MGlobal
import mayaUsd.ufe
import mayaUsd.lib

# 1) Use parallel evaluation, disable evaluation cache.

cmds.evaluationManager(mode="parallel")
cmds.evaluator(enable=False, name="cache")

# 2) Create a simple scene with a prim edited as Maya.

stageShape = mayaUsd.ufe.createStageWithNewLayer('')
stage = mayaUsd.ufe.getStage(stageShape)
prim = stage.DefinePrim("/xform", "Xform")

with mayaUsd.lib.OpUndoItemList():
    mayaUsd.lib.PrimUpdaterManager.editAsMaya(f"{stageShape},/xform")

# 3) Register a Python listener (similar to a python GUI tool).

def dummyNoticeHandler(*_):
    return

listener = Tf.Notice.Register(Usd.Notice.StageEditTargetChanged, dummyNoticeHandler, stage)

# 4) Force evaluation from Python (similar to cmds.mayaUSDExport).

MGlobal.viewFrame(1)
MGlobal.viewFrame(2)

# -->  Maya freezes.

Expected behavior

Maya should evaluate the scene without freezing.

Additional context

Inspecting the maya process thread backtraces suggests a deadlock, where:

• The main thread

  • executes the viewFrame python command
  • waits for parallel evaluation to finish.
  • is very likely still holding the Python GIL.

• A worker thread

  • during MayaUsdProxyShapeBase::compute / ProxyAccessor::compute.
  • changes the stage edit-target using pxr::UsdEditContext
  • tries to acquire the Python GIL to deliver the edit-target notice (Usd.Notice.StageEditTargetChanged) to Python handlers.

The main thread

Thread 1 (Thre07f547632ab8b in sched_yield () at /lib64/libc.so.6
#1  0x00007f5479374ed5 in em::ThreadRun::run(std::function<void ()>) () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#2  0x00007f5479336bbf in tbb::detail::d1::task_arena_function<em::ParallelExecution::evaluateImplementation()::{lambda()#1}, void>::operator()() const () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#3  0x00007f54792a020e in tbb::detail::r1::isolate_within_arena(tbb::detail::d1::delegate_base&, long) () at /usr/local/autodesk/maya2026-x64/lib/libtbb.so.12
#4  0x00007f547932d5fa in em::ParallelExecution::evaluateImplementation() () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#5  0x00007f547936c184 in em::EvaluationGraph::evaluate(bool) () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#6  0x00007f547a404819 in TevaluationManager::evaluate(TdgContext&, bool) () at /usr/local/autodesk/maya2026-x64/lib/libDependEngine.so
#7  0x00007f547a3fc70b in  () at /usr/local/autodesk/maya2026-x64/lib/libDependEngine.so
#8  0x00007f547a4a825e in TdependGraph::setTime(TanimTime const&, bool) () at /usr/local/autodesk/maya2026-x64/lib/libDependEngine.so
#9  0x00007f5482c0ab5d in Autodesk::Maya::OpenMaya20260000::MGlobal::viewFrame(Autodesk::Maya::OpenMaya20260000::MTime const&) () at /usr/local/autodesk/maya2026-x64/lib/libOpenMaya.so
#10 0x00007f5482c0acb5 in Autodesk::Maya::OpenMaya20260000::MGlobal::viewFrame(double) () at /usr/local/autodesk/maya2026-x64/lib/libOpenMaya.so
#11 0x00007f5424182841 in viewFrame(PyObject*, PyObject*) (arg=0x7f546f1a63e8 <_PyRuntime+1800>) at /mnt/data/jenkins/workspace/maya/src/Maya/src/OpenMayaPython/src/OpenMaya/Global/MPyMGlobal.cpp:1054
#12 0x00007f546ee2e8ad in cfunction_vectorcall_O (func=0x7f5424c776a0, args=0x7f542c7bf078, nargsf=<optimized out>, kwnames=<optimized out>) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Include/cpython/methodobject.h:52
#13 0x00007f546edee444 in _PyObject_VectorcallTstate (kwnames=0x6ee361f8, nargsf=<optimized out>, args=0x7f542c7bf078, callable=0x7f5424c776a0, tstate=0x7f546f1ce698 <_PyRuntime+166328>) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Include/internal/pycore_call.h:92
#14 0x00007f546edee444 in PyObject_Vectorcall (callable=callable@entry=0x7f5424c776a0, args=args@entry=0x7f542c7bf078, nargsf=<optimized out>, kwnames=kwnames@entry=0x0) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Objects/call.c:299
#15 0x00007f546ed9ddac in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/ceval.c:7307
#16 0x00007f546eeb9281 in _PyEval_EvalFrame (throwflag=0, frame=0x7f542c7bf020, tstate=0x7f546f1ce698 <_PyRuntime+166328>) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Include/internal/pycore_ceval.h:73
#17 0x00007f546eeb9281 in _PyEval_Vector (tstate=tstate@entry=0x7f546f1ce698 <_PyRuntime+166328>, func=func@entry=0x7f53034fbce0, locals=locals@entry=0x7f54253db000, args=args@entry=0x0, argcount=argcount@entry=0, kwnames=kwnames@entry=0x0) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/ceval.c:6434
#18 0x00007f546eeb9326 in PyEval_EvalCode (co=0x7f52ca4f47b0, globals=<optimized out>, locals=0x7f54253db000) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/ceval.c:1148
#19 0x00007f547a7053c6 in TpythonInterpreter::executeCommand(Tstring const&, bool, bool, TinterpreterResult*) () at /usr/local/autodesk/maya2026-x64/lib/libCommandEngine.so

The maya EM worker thread

Thread 58 (Thread 0x7f53957fe700 (LWP 386404)):
#0  0x00007f54766d6878 in pthread_cond_timedwait@@GLIBC_2.3.2 () at /lib64/libpthread.so.0
#1  0x00007f546eeb839f in PyCOND_TIMEDWAIT (us=<optimized out>, mut=0x7f546f1a5e98 <_PyRuntime+440>, cond=0x7f546f1a5e68 <_PyRuntime+392>) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/condvar.h:73
#2  0x00007f546eeb839f in take_gil (tstate=tstate@entry=0x7f5234002bc0) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/ceval_gil.h:231
#3  0x00007f546eeb888e in PyEval_RestoreThread (tstate=tstate@entry=0x7f5234002bc0) at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/ceval.c:521
#4  0x00007f546eef6bf8 in PyGILState_Ensure () at /mnt/data/jenkins/workspace/3dsmax-conan-recipes@tmp/.conan/data/cpython/3.11.9/3dsmax/testing/build/b9d388bf5e1f3cb7483a70ee89d43fe322c022b2/src/Python/pystate.c:1712
#5  0x00007f52f1e11215 in pxrInternal_v0_24_11__pxrReserved__::TfPyLock::Acquire() () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_tf.so
#6  0x00007f52d9b6eefe in  () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/python/pxr/Tf/_tf.so
#7  0x00007f52d9b6e7d9 in  () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/python/pxr/Tf/_tf.so
#8  0x00007f52f1dc708b in pxrInternal_v0_24_11__pxrReserved__::Tf_NoticeRegistry::_Deliver(pxrInternal_v0_24_11__pxrReserved__::TfNotice const&, pxrInternal_v0_24_11__pxrReserved__::TfType const&, pxrInternal_v0_24_11__pxrReserved__::TfWeakBase const*, void const*, std::type_info const&, std::vector<pxrInternal_v0_24_11__pxrReserved__::TfWeakPtr<pxrInternal_v0_24_11__pxrReserved__::TfNotice::Probe>, std::allocator<pxrInternal_v0_24_11__pxrReserved__::TfWeakPtr<pxrInternal_v0_24_11__pxrReserved__::TfNotice::Probe> > > const&, std::pair<std::__cxx11::list<pxrInternal_v0_24_11__pxrReserved__::TfNotice::_DelivererBase*, std::allocator<pxrInternal_v0_24_11__pxrReserved__::TfNotice::_DelivererBase*> >*, std::_List_iterator<pxrInternal_v0_24_11__pxrReserved__::TfNotice::_DelivererBase*> > const&) () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_tf.so
#9  0x00007f52f1dc8225 in pxrInternal_v0_24_11__pxrReserved__::Tf_NoticeRegistry::_Send(pxrInternal_v0_24_11__pxrReserved__::TfNotice const&, pxrInternal_v0_24_11__pxrReserved__::TfType const&, pxrInternal_v0_24_11__pxrReserved__::TfWeakBase const*, void const*, std::type_info const&) () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_tf.so
#10 0x00007f52f1e0f42f in pxrInternal_v0_24_11__pxrReserved__::TfNotice::_Send(pxrInternal_v0_24_11__pxrReserved__::TfWeakBase const*, void const*, std::type_info const&) const () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_tf.so
#11 0x00007f52f30f828f in pxrInternal_v0_24_11__pxrReserved__::UsdStage::SetEditTarget(pxrInternal_v0_24_11__pxrReserved__::UsdEditTarget const&) () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_usd.so
#12 0x00007f52f307d018 in pxrInternal_v0_24_11__pxrReserved__::UsdEditContext::UsdEditContext(pxrInternal_v0_24_11__pxrReserved__::TfWeakPtr<pxrInternal_v0_24_11__pxrReserved__::UsdStage> const&, pxrInternal_v0_24_11__pxrReserved__::UsdEditTarget const&) () at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/lib/libusd_usd.so
#13 0x00007f52f9fbc21b in MayaUsd_v0::ComputeContext::ComputeContext(MayaUsd_v0::ProxyAccessor&, Autodesk::Maya::OpenMaya20260000::MObject const&, bool) (this=0x7f53957f5bb0, accessor=..., ownerNode=..., useTargetLayer=<optimized out>) at /usr/local/build/maya-usd/deps/OpenUSD/maya2026-x64/include/pxr/base/tf/refPtr.h:1093
#14 0x00007f52f9fbb18e in MayaUsd_v0::ProxyAccessor::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&, bool) (this=0x16398c00, plug=..., dataBlock=..., useTargetLayer=<optimized out>) at /u/local/src/maya-usd/lib/mayaUsd/nodes/proxyAccessor.cpp:505
#15 0x00007f52f9fcbcb8 in MayaUsd_v0::ProxyAccessor::compute(std::unique_ptr<MayaUsd_v0::ProxyAccessor, std::default_delete<MayaUsd_v0::ProxyAccessor> > const&, Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&, bool) (useTargetLayer=<optimized out>, dataBlock=..., plug=..., accessor=std::unique_ptr<MayaUsd_v0::ProxyAccessor> = {...}) at /u/local/src/maya-usd/lib/mayaUsd/nodes/proxyAccessor.h:147
#16 0x00007f52f9fcbcb8 in pxrInternal_v0_24_11__pxrReserved__::MayaUsdProxyShapeBase::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&) (this=this@entry=0x20bd6d20, plug=..., dataBlock=...) at /u/local/src/maya-usd/lib/mayaUsd/nodes/proxyShapeBase.cpp:716
#17 0x00007f52fa5869a3 in MayaUsd_v0::ProxyShape::compute(Autodesk::Maya::OpenMaya20260000::MPlug const&, Autodesk::Maya::OpenMaya20260000::MDataBlock&) (this=0x20bd6d20, plug=..., dataBlock=...) at /u/local/src/maya-usd/plugin/adsk/plugin/ProxyShape.cpp:109
#18 0x00007f5482c8a75b in THsurfaceShape::computePlug(Tplug const&, TdataBlock&) () at /usr/local/autodesk/maya2026-x64/lib/libOpenMaya.so
#19 0x00007f547a40991e in TevaluationNode::evaluateDirtyPlugs(TdataBlockDG*, TevaluationNode::TdirtyPlugType) () at /usr/local/autodesk/maya2026-x64/lib/libDependEngine.so
#20 0x00007f547a40b33c in TevaluationNode::evaluate(em::SyncMessage const&) () at /usr/local/autodesk/maya2026-x64/lib/libDependEngine.so
#21 0x00007f54793329f0 in em::SchedulingTask::Spawner::execute(em::SchedulingNode&) () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#22 0x00007f5479332dd9 in em::SchedulingGraph::executeTheGraph(tbb::detail::d1::task_group&, em::SyncMessage const&) () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#23 0x00007f5479332e10 in em::ParallelExecution::taskEvaluation() () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#24 0x00007f5479332e49 in std::_Function_handler<void (), em::ParallelExecution::evaluateImplementation()::{lambda()#1}::operator()() const::{lambda()#1}>::_M_invoke(std::_Any_data const&) () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#25 0x00007f54793742fb in std::thread::_State_impl<std::thread::_Invoker<std::tuple<em::ThreadRun::ThreadRun()::{lambda()#1}> > >::_M_run() () at /usr/local/autodesk/maya2026-x64/lib/libEvaluationManager.so.1.1.27.0
#26 0x00007f547717eb23 in execute_native_thread_routine () at /lib64/libstdc++.so.6
#27 0x00007f54766d01ca in start_thread () at /lib64/libpthread.so.0
#28 0x00007f547632b8d3 in clone () at /lib64/libc.so.6

Specs (if applicable):

• Rocky Linux 8.10
• gcc 11.2.1
• Maya 2026
• Maya USD dev at 336c878
• OpenUSD v24.11

[AramAzhari-adsk]

Hello @jufrantz ,

Thanks for your detailed report. I am able to reproduce the issue on my side as well. Just curious, what is the reason behind turning off the evaluation caching? It seems that the freeze doesn't occur there. Is this a production scenario or is it just to make the repro steps easier?

We can investigate if things can be improved in terms of notification spamming but I highly recommend keeping the evaluation caching on if possible.

Regards,
Aram

[jufrantz]

Hello @AramAzhari-adsk,

Thanks for your answer.

I disable evaluation caching in the repro case because background evaluation can easily hide the issue. I think this happens because mayaUsdProxyShape is evaluated before we call MGlobal.viewFrame, and the USD scene update occurs at a different time while the cache is being restored. For this repro, setting the Cache Fill preference to Playback only might have been enough instead of fully disabling caching, but I did not test that.

In production, we also strongly recommend that users keep background caching enabled, and we expect most animators use it for productivity. However, we cannot enforce it. It is a user preference and very easy to toggle in the Maya GUI, and we have hundreds of Maya users with different workflows.

• Some departments (non-animators) do not benefit from it and prefer to disable it.
• Evaluation caching is not compatible with all scenes; some nodes disable it automatically.
• Some production scenes are very large with many and/or fat animated geometries, large timeline range (e.g. scenes with mulitple shots), and may require too much memory for caching.