[release/9.0-staging] [mono] Missing memory barrier leads to crash in multi-threaded scenarios #113740
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Co-authored-by: Aleksey Kliger (λgeek) <alklig@microsoft.com>
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Backport of #113140 to release/9.0-staging
/cc @kg
Customer Impact
In specific multithreaded scenarios, a null pointer is dereferenced by the runtime due to an initialization race condition, causing a crash. A customer observed this on Android. This does not expose uninitialized memory, just zeroed memory that hasn't been filled in yet.
Regression
47c09fa (.NET 7)
Testing
We are relying on CI and customer validation for this as they are embedding an application with Unreal Engine. It is unclear why the race condition was overlooked originally.
Risk
Low risk. This adds asserts for deterministic early failure (especially on targets where it is legal to dereference null, like wasm) and adds an additional memory barrier in a specific place, so it should not be possible for it to cause an undesirable behavior regression.
The memory barrier could theoretically regress performance, but this is initialization code that runs rarely so the impact should be basically nil.