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Redline: concurrent calls on one Instance silently return wrong values

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
java, wasm
Domain
backend, compilers

Research direction

The issue is in the Redline backend's machine state, specifically the argsBuffer, callDepth, TRAP_CODE, and pendingException fields which are shared across threads. Start by examining the machine class and the host-call argument passing mechanism. The goal is to implement a thread-owner check on the outermost call to turn silent corruption into an error, as suggested. Look for the compiled calling convention to understand the context pointer.

Written by the indexing model from the issue text.

Description

Calling one native Instance from several threads returns wrong results with no error. The other backends do not do this: bytecode is correct, and the interpreter throws. Redline is the only one that answers quietly and wrongly.

Eight threads, 2000 calls each, on a shared instance of host-import-roundtrip.wat.wasm, whose callTakeI32 must always return -1:

Backend Failed calls Wrong results
Interpreter 16000 / 16000 0
Bytecode 0 / 16000 0
Redline (jffi) 5318 / 16000 783

Every Redline failure is TrapException: call stack exhausted. The 783 wrong results carried no error at all.

Cause

Per-call state lives in the machine, shared by every thread that enters it, rather than per thread.

  • argsBuffer is one buffer per machine, and host-call arguments pass through it, so one thread reads another's argument. takeI32 returns what it was handed, which is where the wrong values come from.
  • callDepth is an unsynchronised int, so outermostCall is missed, STACK_LIMIT is never re-anchored, and the stack guard fires against a stale limit.
  • TRAP_CODE and pendingException are shared too, so one thread can see and clear another's trap.

Instances are not thread-safe in general and this may simply be unsupported usage. It is filed for the failure mode rather than the usage: silent wrong answers instead of an error.

Suggested fix

Cheap, and worth doing on its own: compare-and-set an owner thread on the outermost call and throw if another thread is already inside. One atomic operation per outermost call, and it turns silent corruption into an actionable error, which is what the interpreter already does in effect.

Real: per-thread context and argument buffers, and a per-thread call depth. Bigger, because the context pointer is part of the compiled calling convention.

Note on #201

The watchdog fix does not cause this, but it does make it visible. Before it, the same probe gave 0 to 3 wrong results rather than 783: starting a thread per call serialised callers enough that most trapped before they could race. The data race is older than the watchdog work; what went away is the throttle that was hiding it.

🤖 Generated with Claude Code

Dominant language
Java
Stars
310
Forks
23
Avg merge
2d 8h
Merged PRs (30d)
33

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