Game will hang completely, the only out is to close. Could you prepare a fix for 5.8 that we can cherry-pick meanwhile?
Callstack.png(440 KB)
Steps to Reproduce
Looks like there’s a deadlock in UE physical animation code. Repro rate is relatively low but the game will hang, so critical for us.
See screenshot attached. During PhysicalAnimationComponent::Tick Game Thread locks Chaos Physics scene (W) and waits for BlendPhysicsTask to finish. Meanwhile BlendPhysicsTask attempts to take Chaos Physics scene lock (R) which is already taken by the game thread. After searching Unreal issue tracker I managed to find that a very similar issue was reported back in 2023. (“Very similar” in the sense that it’s the same deadlock, but Game thread callstack of reporter is slightly different from ours, most likely because it was reported using UE 5.3)
The reporter mentions that it reproduces only in PIE for them and that it happens very rarely. The issue was last updated in October 2025 and target fix version is 5.8
Hi Pierre and thank you for the question.
This is an experimental class, and we aren’t currently working on it is the fast answer. If you disable parallel blending this should go away.
Best
Geoff Stacey
Developer Relations
EPIC Games