ampere-kernel-decoders/phase4_plan_iter6_v3.md

Phase 4 — iter6 v3 plan (KASAN+KCSAN delta on v2 substrate)

Plan ID: iter6-v3-kasan-kcsan Author: Claude Opus 4.7 Status: Pending Phase 5 architect delta-review Cross-ref: phase4_plan_iter6_postmortem_v2.md (commit 1656f84, executed), iter6_v2_attempt2_close.md (commit 53e2477)

Why v3

v2 attempt-2 found: 0004 alone passes headless HEVC clean, but wedges silently ~10 min later under GPU compositor + concurrent vb2 traffic. PROVE_LOCKING+9 lock-debug flags did NOT fire before the wedge → not a classical lock-order inversion lockdep models.

Refined hypothesis (4 candidates) needs different sanitizers:

• H1' cross-context wait not lockdep-visible → KCSAN
• H2 use-after-free / double-signal of dma_fence → KASAN
• H3 race in fence alloc/init → KCSAN
• H4 panthor consumer-side bug → headless-only repro path

Delta from v2 plan

All v2 pre-flight + lockdep config + ramoops setup is preserved (already executed and validated). v3 adds:

Kernel config additions (step 1.2-DELTA)

./scripts/config --enable KASAN --enable KASAN_GENERIC \
                 --enable KASAN_OUTLINE \
                 --enable KCSAN --enable KCSAN_REPORT_ONCE_IN_MS=2000 \
                 --enable DEBUG_PREEMPT \
                 --enable DEBUG_OBJECTS --enable DEBUG_OBJECTS_RCU_HEAD \
                 --enable SLUB_DEBUG --enable SLUB_DEBUG_ON \
                 --enable PAGE_POISONING --enable PAGE_POISONING_NO_SANITY
CONFIG_LOCALVERSION="-lockdep-kasan"

Notes:

• KASAN_GENERIC (not SW_TAGS) — generic mode works on all aarch64
• KASAN_OUTLINE (not INLINE) — slower but smaller binary, supports more features
• KCSAN_REPORT_ONCE_IN_MS=2000 — rate-limit reports so console doesn't drown
• SLUB_DEBUG_ON — enable all SLUB debug at boot (poisoning, redzone, owner-tracking)
• DEBUG_OBJECTS — catches double-init / use-after-free of registered kernel objects (dma_fences are registered)

Build (step 1.3-DELTA)

time make -j8 Image modules dtbs — estimated 75-90 min (KASAN+KCSAN compile slow)

Install (step 1.4-1.7-DELTA)

• Modules install to /lib/modules/7.0.0-rc3-lockdep-kasan+/ (separate via LOCALVERSION)
• Image installs to /boot/firmware/Image-7.0.0-rc3-lockdep-kasan+
• Initramfs for new release
• Copy lockdep DTB (with ramoops node) to /boot/firmware/rk3588-coolpi-cm5-genbook.dtb-7.0.0-rc3-lockdep-kasan+
• ADD third extlinux label arch_devices_lockdep_kasan. Keep arch_devices (vanilla) as default. Keep arch_devices_lockdep (without KASAN) as a fallback if KASAN kernel doesn't boot
• User picks arch_devices_lockdep_kasan at U-Boot menu (requires HDMI for first boot — OR temporarily flip default for the test)

Bisect-apply (step 2.x-DELTA)

Same as v2: 0004 → 0005 → 0006 → 0007-v2. Between each: reboot + smoke + harvest journal/pstore for KASAN/KCSAN reports.

Critical addition for step 2.1: after the headless smoke test passes, intentionally trigger GPU compositor activity to reproduce the v2 wedge condition:

1. Log into plasma wayland at SDDM prompt (manual, physical)
2. Start a video that drives both GPU compositor + V4L2 decode concurrently. E.g.: mpv --hwdec=vaapi-copy ~/measurements/encoded/bbb_60s_720p.hevc.mp4 in the plasma session
3. Watch for ~2 minutes (v2 wedge timing was ~10 min, but KASAN/KCSAN should catch earlier)
4. From ssh: tail -f /sys/fs/pstore/ AND journalctl -k -f for KASAN / KCSAN / BUG: KASAN / data-race / use-after-free reports
5. If a report fires, capture it then reboot back to lockdep-non-kasan kernel for analysis

Risk register additions vs v2

#	Risk	Mitigation
R7	KASAN+KCSAN combined boot too slow → systemd timeout → boot failure	Multiple fallback labels in extlinux: arch_devices_lockdep_kasan (new), arch_devices_lockdep (no kasan), arch_devices (default vanilla). User picks at U-Boot. systemd unit timeouts may need extension via DefaultTimeoutStartSec= if observed
R8	KASAN region overlaps with our ramoops carve-out (0x10000000-0x100fffff)	KASAN shadow region uses high VA, not low PA. Carve-out is from real physical RAM, KASAN shadow is in vmalloc space. No conflict expected — verify with /proc/iomem post-boot
R9	KASAN tags trigger false positives on pre-existing kernel code (e.g. panel-edp's WARN) → noise drowns real signal	KASAN reports are distinctive ("BUG: KASAN:" prefix). Grep filtering should isolate. If false-positive volume is overwhelming, switch to KASAN_SW_TAGS or selectively disable instrumentation per-file
R10	GPU-compositor wedge reproduction requires physical login (no remote-wayland start)	User has physical access to ampere for this test step (per v2 plan A4 conditional). Time-boxed: if wedge not reproduced in ~10 min of compositor activity, declare iter6 v3 inconclusive

Estimated wall-time

Phase	Time
Pre-flight + config delta	5 min (config additions only, no new pre-flight)
KASAN+KCSAN kernel build	75-90 min
Install + reboot + lockdep-kasan baseline	10 min
Apply 0004 + module rebuild + install + reboot + headless smoke	15 min
GPU-compositor concurrent test + monitor for KASAN/KCSAN reports	15 min
Subtotal for 0004 verdict	~120 min wall-time
0005/0006/0007 follow-ups if 0004 verdict is "clean under KASAN"	+30 min each

Exit conditions

Success: KASAN or KCSAN report fires during step 2.1's GPU-compositor test, pinpointing the bug class + responsible code. → file kernel-agent issue, fix in 0004 v2 OR upstream, retry.

Partial: 0004 v3 (with KASAN) still wedges silently with NO KASAN/KCSAN report. Means bug is OUTSIDE the sanitizer-instrumented memory (e.g. firmware blob, GPU hardware state machine). Switch to serial console for live trace (final fallback).

Failure: KASAN+KCSAN kernel doesn't boot at all → fall back to lockdep kernel, declare iter6 line closed, move iter7 to non-fence cache-coherency angle.