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The producer side (`mmio_region.rs:78`, the guest's CP_RB_WPTR MMIO write callback) uses `Ordering::Release` so any ring-memory writes the guest performed before bumping WPTR are visible to a paired `Acquire`-load on the consumer. The consumer here at `sync_with_mmio` was using `Ordering::Relaxed` for both the WPTR load and the RPTR mirror store — leaving the Release/Acquire pairing broken. Under `--parallel`, this broken pairing means the GPU worker can observe a fresh WPTR value while still reading stale ring-memory contents at the corresponding offsets — garbage PM4 packets. The audit's M11 grid run confirmed --parallel is non-deterministic beyond the documented `packets` ±5% noise; this fix is one strand of that. Symmetric fix on the RPTR mirror store: Release pairs with any guest-side Acquire-load of CP_RB_RPTR for ring-writeback bookkeeping. Verification at -n 100M lockstep: swaps: 2 → 2 (unchanged) draws: 0 → 0 (unchanged) packets: ~60M (within noise) Tests: 149 (no count change; this is a memory-ordering correctness fix, not a behavioral change visible at the digest level in lockstep). Closes GPUBUG-006 (P1). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>