fix(barrier): resolve cuDSS mtlayer path at runtime by ramakrishnap-nv · Pull Request #1982 · NVIDIA/cuopt

ramakrishnap-nv · 2026-09-23T16:05:12Z

Re-derives the cuDSS threading-layer plugin path from wherever libcudss.so.0 actually loaded from, instead of trusting the compile-time-baked absolute path (which can go stale, e.g. an unrelocated conda/rattler-build sandbox path). Also drops the alarming "to avoid a possible crash during teardown" wording from the fallback warning.

The compiled-in absolute path to cuDSS's prebuilt threading-layer plugin is resolved at CMake configure time and can go stale once installed elsewhere (e.g. an unrelocated conda/rattler-build sandbox path), causing dlopen to fail. Re-derive it from wherever libcudss.so.0 actually loaded from at runtime instead. Also drops "to avoid a possible crash during teardown" from the fallback warning. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

coderabbitai · 2026-09-23T16:10:05Z

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📝 Walkthrough

Walkthrough

When the custom cuDSS threading layer is disabled, CMake retains the previously computed library basename. The pin-failure log no longer mentions a possible teardown crash.

Changes

cuDSS threading-layer configuration

Layer / File(s)	Summary
Threading-layer configuration and pin-failure log `cpp/CMakeLists.txt`, `cpp/src/barrier/sparse_cholesky.cuh`	When the custom cuDSS threading layer is disabled, CMake retains the previously computed library basename. The pin-failure log no longer mentions a possible teardown crash.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 18f24

Some installations may fall back to single-threaded cuDSS instead of using the configured threading layer. Confirm the supported library layouts or make the default plugin path runtime-resolvable before merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: resolving the cuDSS mtlayer path at runtime.
Description check	✅ Passed	The description directly explains the runtime path resolution change and the warning text update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
In `@cpp/src/barrier/sparse_cholesky.cuh`:
- Line 57: Update the address passed to dladdr in the cudssCreateMg lookup so it
comes from the loaded cuDSS object rather than the caller’s PLT entry; preserve
the existing library-path handling using the resolved cuDSS implementation
address.

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cpp/src/barrier/sparse_cholesky.cuh

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&cudssCreateMg can be our own PLT stub rather than the real symbol, which dladdr would attribute to cuOpt's own module instead of libcudss.so.0. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

github-actions · 2026-09-23T16:19:36Z

CI Test Summary

✅ All 32 test job(s) passed.

chris-maes · 2026-09-23T18:01:50Z


    const char* cudss_mt_lib_file = nullptr;
-    char* env_value               = std::getenv("CUDSS_THREADING_LIB");
+    std::string resolved_lib_file;


Can this move inside the `if (cudss_mt_lib_file[0] == '/')?

Good catch — the separate string couldn't safely move inside that if (its pointer needs to outlive the block). Removed the two-variable pattern entirely: cudss_mt_lib_file is now a single std::string reassigned in place, no raw pointer/lifetime issue anymore.

chris-maes · 2026-09-23T18:02:17Z

      cudss_mt_lib_file = env_value;
    } else if (CUDSS_MT_LIB_FILE_NAME != nullptr) {
      cudss_mt_lib_file = CUDSS_MT_LIB_FILE_NAME;
+      // Absolute path (cuDSS's own mtlayer) can go stale; re-derive at runtime.


What does this mean? "can go stale"?

Reworded — it means CUDSS_MT_LIB_FILE_NAME is a path resolved at CMake configure time from cuDSS's build-time install location, which can differ from (or not exist at) its actual install location wherever the package ends up (e.g. an unrelocated build sandbox path).

…ify comment Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

hlinsen · 2026-09-23T21:54:40Z

 }

+// Directory libcudss.so.0 loaded from; cuDSS ships its mtlayer plugin alongside it.
+inline std::string cudss_library_dir()


Suggestion as this seems very complicated for a rare bug:

We could restore filename lookup for conda instead of adding cudss_library_dir(). pin_cudss_threading_layer() loads the plugin first with RTLD_NODELETE, and we pass the same name to cudssSetThreadingLayer() only if pinning succeeds. glibc checks already loaded libraries by name before searching paths. Conda CI used the filename successfully before #1960; is there a supported case where these calls could select different plugins?

You're right, and there's history that backs this up: git blame shows the switch to a full path was a deliberate choice in #1883 specifically so the pin and cudssSetThreadingLayer() resolve the same file -- but per the glibc behavior you linked, a bare filename actually satisfies that more reliably, since both calls hit the same already-loaded soname entry regardless of search path. Switched to keeping the basename in both branches (dropped the whole dladdr/dlsym mechanism) -- much smaller diff.

…overy glibc's dlopen matches an already-loaded library by name before searching paths, so a bare filename resolves the same mapping as the initial pin without needing a build-time-resolved absolute path that can go stale. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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cpp/src/barrier/sparse_cholesky.cuh

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ramakrishnap-nv · 2026-09-28T18:31:28Z

/merge

ramakrishnap-nv requested a review from a team as a code owner September 23, 2026 16:05

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Comment thread cpp/src/barrier/sparse_cholesky.cuh Outdated

ramakrishnap-nv self-assigned this Sep 23, 2026

ramakrishnap-nv added non-breaking Introduces a non-breaking change improvement Improves an existing functionality labels Sep 23, 2026

ramakrishnap-nv added this to the 26.10 milestone Sep 23, 2026

ramakrishnap-nv and others added 2 commits September 23, 2026 11:17

fix review: resolve via dlsym before dladdr to avoid PLT stub

0058b8b

&cudssCreateMg can be our own PLT stub rather than the real symbol, which dladdr would attribute to cuOpt's own module instead of libcudss.so.0. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

trim comments to minimum

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Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

chris-maes reviewed Sep 23, 2026

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address review: single std::string instead of raw-pointer dance, clar…

0cbb7f9

…ify comment Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

hlinsen reviewed Sep 23, 2026

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ramakrishnap-nv requested review from a team as code owners September 23, 2026 22:03

ramakrishnap-nv requested review from Iroy30 and msarahan September 23, 2026 22:03

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Comment thread cpp/src/barrier/sparse_cholesky.cuh

hlinsen approved these changes Sep 28, 2026

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ramakrishnap-nv requested review from gforsyth and removed request for msarahan September 28, 2026 18:32

tmckayus approved these changes Sep 29, 2026

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rapids-bot Bot merged commit a33c811 into main Sep 29, 2026
159 of 161 checks passed

ramakrishnap-nv deleted the fix/cudss-mtlayer-path-resolution branch October 16, 2026 15:12

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