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fix(profiler): eliminate SIGSEGV race on _class_map.clear() in signal handler and dump path - #516
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jbachorik
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May 8, 2026
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…fter-unlock in writeClasses; cap tryLockShared at 5 attempts Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Pull request overview
This PR addresses a native crash (SIGSEGV) in the profiler dump path by enforcing a consistent locking protocol around _class_map access across the dump path, JFR writing, and signal-handler stack walking. It also adds a “chaos harness” workload and CI wiring intended to continuously stress profiler crash surfaces, plus improvements to JFR validation skip reporting.
Changes:
- Make
_class_mapaccesses signal-safe and race-free by using shared/exclusiveSpinLockguards consistently (including a try-lock +bounded_lookup(…, size_limit=0)path in the signal handler). - Extend
SpinLockguard types to support RAII factory methods, and add regression/concurrency gtests for dictionary contracts. - Add a long-running “chaos” harness (new Gradle sourceSet + fat jar) and scheduled GitLab reliability jobs; improve JFR validation “skip reason” propagation.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
| test-validation/validate-jfr-conformance.sh | Reads and reports a concrete skip reason when JFR validation is disabled. |
| gradle/libs.versions.toml | Adds dd-trace-api dependency for the chaos harness (currently via a dynamic version). |
| ddprof-stresstest/src/chaos/README.md | Documents the new chaos harness and its antagonist workloads. |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/Antagonist.java | Defines the harness antagonist interface (start/stop). |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/Main.java | Adds a small CLI runner for selecting antagonists and duration. |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/ThreadChurnAntagonist.java | Adds a thread churn workload to stress teardown vs sampling. |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/VirtualThreadChurnAntagonist.java | Adds a vthread churn workload (reflective, Java 21+). |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/ClassLoaderChurnAntagonist.java | Adds classloader/class-unload churn using ASM-generated classes. |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/AllocStormAntagonist.java | Adds allocation pressure workload (heap + reflective Unsafe native alloc). |
| ddprof-stresstest/src/chaos/java/com/datadoghq/profiler/chaos/TraceContextAntagonist.java | Adds a tracer-driven context churn workload via @Trace. |
| ddprof-stresstest/build.gradle.kts | Adds chaos sourceSet, dependencies, and chaosJar fat-jar task. |
| ddprof-lib/src/test/cpp/dictionary_concurrent_ut.cpp | Adds concurrency/regression gtests covering bounded_lookup and locking discipline. |
| ddprof-lib/src/main/cpp/spinLock.h | Bounds tryLockShared, adds movable RAII guard types used by new factory methods. |
| ddprof-lib/src/main/cpp/profiler.h | Exposes RAII factory methods for acquiring the class-map lock shared/try-shared. |
| ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp | Uses try-shared guard + bounded_lookup(...,0) in the signal-handler walkVM path. |
| ddprof-lib/src/main/cpp/flightRecorder.cpp | Holds a shared guard across writeClasses() iteration to prevent UAF vs clear(). |
| ddprof-lib/src/main/cpp/dictionary.h | Documents the signal-unsafety of inserting lookup() and the bounded_lookup(...,0) contract. |
| .gitlab/reliability/run.sh | Adds a new reliability variant entry-point for the chaos harness. |
| .gitlab/reliability/chaos_check.sh | Implements the chaos run logic (patched agent + chaos jar + allocator variants). |
| .gitlab/reliability/.gitlab-ci.yml | Adds scheduled chaos reliability jobs (amd64/aarch64 matrix) and notification wiring. |
| .gitlab/dd-trace-integration/install-prerequisites.sh | Writes a detailed skip reason and pre-warms the jfr-shell backend. |
| .gitlab/common.yml | Changes get-versions job setup (adds SDKMAN + JDK install, stricter shell flags). |
| .gitlab/build-deploy/.gitlab-ci.yml | Adds a schedule-only chaos:build job to prebuild/publish the chaos jar artifact. |
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CI Test ResultsRun:#28552212991 | Commit:
Summary: Total: 0 | Passed: 0 | Failed: 0 Updated: 2026-07-01 22:40:57 UTC |
…unded variant - ddprof-lib/src/main/cpp/spinLock.h:49 — restored tryLockShared() to spin until exclusive lock released; added tryLockSharedBounded() for signal-handler paths - ddprof-lib/src/main/cpp/spinLock.h:102 — OptionalSharedLockGuard updated to use tryLockSharedBounded() Co-Authored-By: muse <muse@noreply>
jbachorik
commented
May 8, 2026
@chatgpt-codex-connector review |
jbachorik
commented
May 8, 2026
@copilot review |
…(), not tryLockShared() The previous commit added tryLockSharedBounded() and claimed OptionalSharedLockGuard was updated to use it, but the constructor still called tryLockShared() — the spinning (potentially unbounded) variant. Since OptionalSharedLockGuard is used in signal-handler paths, it must use the bounded variant to be async-signal-safe. Agent-Logs-Url: https://github.com/DataDog/java-profiler/sessions/4b50411f-3c52-4938-9d54-a43ada34b94e Co-authored-by: jbachorik <738413+jbachorik@users.noreply.github.com>
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Fixed in commit 9876873. The previous commit added |
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- ddprof-lib/src/main/cpp/spinLock.h:115 — OptionalSharedLockGuard reverted to tryLockShared() (non-spurious) for FlightRecorder hot paths; BoundedOptionalSharedLockGuard added for signal-handler-only callers - ddprof-lib/src/main/cpp/profiler.h:208 — classMapTrySharedGuard() now returns BoundedOptionalSharedLockGuard Co-Authored-By: muse <muse@noreply>
jbachorik
commented
May 11, 2026
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Codex Review: Didn't find any major issues. Another round soon, please! ℹ️ About Codex in GitHubYour team has set up Codex to review pull requests in this repo. Reviews are triggered when you
If Codex has suggestions, it will comment; otherwise it will react with 👍. Codex can also answer questions or update the PR. Try commenting "@codex address that feedback". |
rkennke
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Is the PR missing the promised new tests?
jbachorik
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May 11, 2026
@rkennke 🤦 Good catch. I was rebasing locally and killed the added tests :/ Will fix! |
…ounded unit tests - spinlock_bounded_ut.cpp: 6 tests covering tryLockSharedBounded() and BoundedOptionalSharedLockGuard in isolation (free lock, exclusive-held failure, multiple shared holders, RAII destruction, concurrent stress) - dictionary_concurrent_ut.cpp: update SignalHandlerBoundedLookupVsDumpClear to use BoundedOptionalSharedLockGuard matching the actual hotspotSupport.cpp code path (classMapTrySharedGuard()) Co-Authored-By: muse <muse@noreply>
The tests are now present. Two commits add coverage: 7df7230 — 45454fd — Also fixed the PR description which had stale references ( |
…nalSharedLockGuard Restore OptionalSharedLockGuard in SignalHandlerBoundedLookupVsDumpClear and add SignalHandlerBoundedOptionalLookupVsDumpClear for BoundedOptionalSharedLockGuard, which is what classMapTrySharedGuard() returns in hotspotSupport.cpp. Co-Authored-By: muse <muse@noreply>
rkennke
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It seems slightly inconsistent/confusing that BoundedOptionalSharedLockGuard is non-movable, but others are now movable. With C++17, this is ok with guaranteed copy elision, but it's still mildly confusing.
OptionalSharedLockGuard move constructor looks like not used at all?
Other than that, the changes look good!
jbachorik
commented
May 11, 2026
Addressed — removed unused move constructors from |
… non-movable - SharedLockGuard: remove move ctor, simplify dtor (nullptr guard only served moved-from instances) - OptionalSharedLockGuard: remove move ctor - BoundedOptionalSharedLockGuard: already non-movable, unchanged C++17 guaranteed copy elision handles the factory-return pattern without move constructors. Co-Authored-By: muse <muse@noreply>
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…ufferedDictionary Replaces the SpinLock-guarded Dictionary instances for _class_map, _string_label_map, and _context_value_map with a new TripleBufferedDictionary that eliminates all locking from the read/write fast paths. TripleBufferedDictionary holds three Dictionary buffers cycling through three roles via a generic TripleBufferRotator<T> template: - active — receives new writes (signal handlers + JNI threads), lock-free via CAS - dump — snapshot being read by the dump thread; promoted from old active on rotate() - scratch — two rotations behind active; ready to be cleared lock-free The scratch role exists for safe lock-free reclamation: when a buffer enters that role, at least one full dump cycle has elapsed since it was last in the active or dump role. That grace period is much longer than any signal-handler or JNI-thread can plausibly outlive a stale active pointer, so the buffer can be freed without any explicit drain. bounded_lookup(size_limit=0) is signal-safe (no malloc) and checks the active buffer only — no fallback to older snapshots. Dead code removed: - _class_map_lock (SpinLock) - classMapSharedGuard() / classMapTrySharedGuard() on Profiler - tryLockSharedBounded() / BoundedOptionalSharedLockGuard on SpinLock - spinlock_bounded_ut.cpp / dictionary_concurrent_ut.cpp (subsumed by dictionary_ut.cpp) Motivation: three production crashes (fingerprint v10.DAECC680F0728EAB44F26DB0B91B703F) showed SIGSEGV in std::_Rb_tree_increment via writeCpool → writeClasses → Dictionary::collect, caused by a race between writeClasses and concurrent Dictionary::clear(). PR #516 patched it with a shared-lock that exhausted bounded CAS retries under heavy 100 µs wall-clock load on aarch64, causing class lookups to return -1 and corrupting JFR recordings. This change eliminates the lock entirely. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…ufferedDictionary Replaces the SpinLock-guarded Dictionary instances for _class_map, _string_label_map, and _context_value_map with a new TripleBufferedDictionary that eliminates all locking from the read/write fast paths. TripleBufferedDictionary holds three Dictionary buffers cycling through three roles via a generic TripleBufferRotator<T> template: - active — receives new writes (signal handlers + JNI threads), lock-free via CAS - dump — snapshot being read by the dump thread; promoted from old active on rotate() - scratch — two rotations behind active; ready to be cleared lock-free The scratch role exists for safe lock-free reclamation: when a buffer enters that role, at least one full dump cycle has elapsed since it was last in the active or dump role. That grace period is much longer than any signal-handler or JNI-thread can plausibly outlive a stale active pointer, so the buffer can be freed without any explicit drain. bounded_lookup(size_limit=0) is signal-safe (no malloc) and checks the active buffer only — no fallback to older snapshots. Dead code removed: - _class_map_lock (SpinLock) - classMapSharedGuard() / classMapTrySharedGuard() on Profiler - tryLockSharedBounded() / BoundedOptionalSharedLockGuard on SpinLock - spinlock_bounded_ut.cpp / dictionary_concurrent_ut.cpp (subsumed by dictionary_ut.cpp) Motivation: three production crashes (fingerprint v10.DAECC680F0728EAB44F26DB0B91B703F) showed SIGSEGV in std::_Rb_tree_increment via writeCpool → writeClasses → Dictionary::collect, caused by a race between writeClasses and concurrent Dictionary::clear(). PR #516 patched it with a shared-lock that exhausted bounded CAS retries under heavy 100 µs wall-clock load on aarch64, causing class lookups to return -1 and corrupting JFR recordings. This change eliminates the lock entirely. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…ufferedDictionary Replaces the SpinLock-guarded Dictionary instances for _class_map, _string_label_map, and _context_value_map with a new TripleBufferedDictionary that eliminates all locking from the read/write fast paths. TripleBufferedDictionary holds three Dictionary buffers cycling through three roles via a generic TripleBufferRotator<T> template: - active — receives new writes (signal handlers + JNI threads), lock-free via CAS - dump — snapshot being read by the dump thread; promoted from old active on rotate() - scratch — two rotations behind active; ready to be cleared lock-free The scratch role exists for safe lock-free reclamation: when a buffer enters that role, at least one full dump cycle has elapsed since it was last in the active or dump role. That grace period is much longer than any signal-handler or JNI-thread can plausibly outlive a stale active pointer, so the buffer can be freed without any explicit drain. bounded_lookup(size_limit=0) is signal-safe (no malloc) and checks the active buffer only — no fallback to older snapshots. Dead code removed: - _class_map_lock (SpinLock) - classMapSharedGuard() / classMapTrySharedGuard() on Profiler - tryLockSharedBounded() / BoundedOptionalSharedLockGuard on SpinLock - spinlock_bounded_ut.cpp / dictionary_concurrent_ut.cpp (subsumed by dictionary_ut.cpp) Motivation: three production crashes (fingerprint v10.DAECC680F0728EAB44F26DB0B91B703F) showed SIGSEGV in std::_Rb_tree_increment via writeCpool → writeClasses → Dictionary::collect, caused by a race between writeClasses and concurrent Dictionary::clear(). PR #516 patched it with a shared-lock that exhausted bounded CAS retries under heavy 100 µs wall-clock load on aarch64, causing class lookups to return -1 and corrupting JFR recordings. This change eliminates the lock entirely. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
What does this PR do?:
Fixes a SIGSEGV in `Profiler::dump` caused by a three-way race on `_class_map`:
Motivation:
Crash reports showing SIGSEGV with frames `Dictionary::clear` ← `Profiler::dump` ← `JavaProfiler.dump0`.
Changes:
Additional Notes:
How to test the change?:
For Datadog employees:
credentials of any kind, I've requested a review from `@DataDog/security-design-and-guidance`.
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