The Phase-C VdSwap PM4 ring path (commit 82f3d61) emits two
"PM4_XE_SWAP not consumed by drain" warnings when running:
exec sylpheed.iso --ui --quiet --halt-on-deadlock \
--parallel --reservations-table
Lockstep -n 100M never trips it. Two distinct race windows:
(a) Inline backend (--ui forces it): drain(mem, 4096) hit its
fixed packet cap before reaching the PM4_XE_SWAP we'd just
injected at the WPTR tail. With 6 CPU threads, the ring
accumulates >4096 packets between vd_swap callbacks.
(b) Threaded backend (--parallel without --ui): the worker's
DrainFence handler has a 900 ms deadline and game-batched
IBs (8-10 M packets observed) keep it from reaching the
tail in any reasonable budget. If the worker eventually
drained past the injected packet later, the safety-net
direct notify would double-count.
Three changes:
* gpu_system.rs: new `drain_until_wptr(target, time_budget)`
draining by the canary `WorkerThreadMain` predicate
(read_offset != target) instead of a fixed packet count.
900 ms deadline mirrors the threaded DrainFence handler.
* handle.rs: inline `drain_to_current_wptr` switches to
`drain_until_wptr`. DrainFence handler publishes the digest
mirror BEFORE replying so the CPU's post-drain
`digest_snapshot` sees fresh stats.
* exports.rs (vd_swap): skip the PM4 ring injection
unconditionally and route swap notification through
`notify_xe_swap` directly. Tail-injection is unreliable
under --parallel for both backends. The slot-0
fetch-constant patch is deferred (GPUBUG-FETCH-PATCH-001);
draws=0 today so a stale slot 0 has no observable effect.
Verification:
* cargo test --workspace --release: 556 passing (unchanged).
* Lockstep -n 100M --stable-digest: bit-identical to
pre-fix master HEAD aa3f1d3.
{instructions:100000002, imports:987685, unimpl:0, draws:0,
swaps:2, ...}
* check --parallel --reservations-table -n 30M: 0 warnings
(was 2). swaps=2.
* exec --gpu-inline --parallel --reservations-table -n 30M:
0 warnings (was 2 with drained=8M-10M observed). swaps=2.
Audit IDs: GPUBUG-DRAIN-001 (closed),
GPUBUG-FETCH-PATCH-001 (filed, deferred).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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