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>
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