# concurrency-debugging

A skill that walks the agent through ThreadSanitizer reports, Helgrind lock-order graphs and stuck threads in GDB. Useful when a threaded program hangs or crashes.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 524 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/debuggers/concurrency-debugging/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/concurrency-debugging

## What it does

The skill lays out a workflow for concurrency bugs: how to read a TSan report, how to spot lock-order violations with Helgrind, how to inspect blocked threads in GDB. TSan builds go through `clang -fsanitize=thread -g -O1` or `gcc -fsanitize=thread -g -O1`; Helgrind runs as `valgrind --tool=helgrind --log-file=helgrind.log ./prog`.

There is a table of common races with fixes: unsynchronized reads and writes on a global, double-checked locking without `atomic`, `+=` on a shared counter, a container mutated while iterated. A separate section covers `std::atomic` misuse — why `compare_exchange_strong` is needed where `if (counter == 0) counter = 1` leaves a TOCTOU race, and why `memory_order_relaxed` fails as a readiness flag. Happens-before gets its own pass: sequenced-before inside a thread, synchronizes-with across threads, thread creation and join, mutex unlock and lock.

For Rust the skill points at `Arc<Mutex<T>>` and compile-time guarantees. One thing to know: the reasoning and examples stick to C++ and Rust, and the tooling shown is `clang`, `gcc`, `valgrind`, `gdb` — on another toolchain you will be translating the examples yourself.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill concurrency-debugging -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/concurrency-debugging/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/concurrency-debugging/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill concurrency-debugging -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/concurrency-debugging/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill concurrency-debugging -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/concurrency-debugging/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills concurrency-debugging
```

2. **Manually: put the files in place**
- File: `.github/skills/concurrency-debugging/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill concurrency-debugging -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/debuggers/concurrency-debugging
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/concurrency-debugging/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill concurrency-debugging -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/debuggers/concurrency-debugging/SKILL.md

## When to use it
- read a ThreadSanitizer report and locate the conflicting accesses
- catch a lock-order violation before it turns into a deadlock
- find which threads are blocked and which mutex each one holds
- spot a TOCTOU race in a compound `std::atomic` operation
- work out happens-before between threads in C++ or Rust

## Example prompts
- ThreadSanitizer flagged a data race — help me read the report and fix it
- My program hangs on threads, show me how to find the deadlock in GDB
- Check whether my memory_order choices are correct here

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: b4a3d72217b4dd31.

BEGIN THIRD-PARTY FILE b4a3d72217b4dd31
````text
---
name: concurrency-debugging
description: Concurrency debugging skill for diagnosing data races and deadlocks. Use when reading TSan race reports, debugging deadlocks with GDB thread inspection, analyzing lock-order graphs with Helgrind, identifying std::atomic misuse patterns, or reasoning about happens-before in C++ and Rust. Activates on queries about data races, TSan reports, deadlocks, Helgrind, lock ordering, thread sanitizer output, or atomic ordering issues.
---

# Concurrency Debugging

## Purpose

Guide agents through diagnosing and fixing concurrency bugs: reading ThreadSanitizer race reports, using Helgrind for lock-order analysis, detecting deadlocks with GDB thread inspection, identifying common `std::atomic` misuse patterns, and applying happens-before reasoning in C++ and Rust.

## Triggers

- "ThreadSanitizer reported a data race — how do I read the report?"
- "My program deadlocks — how do I debug it?"
- "How do I use Helgrind to find threading bugs?"
- "Am I using std::atomic correctly?"
- "How does happens-before work in C++ memory ordering?"
- "How do I find which threads are deadlocked in GDB?"

## Workflow

### 1. ThreadSanitizer (TSan) — race detection

```bash
# Build with TSan
clang -fsanitize=thread -g -O1 -o prog main.c
# or GCC
gcc -fsanitize=thread -g -O1 -o prog main.c

# Run (TSan intercepts memory accesses at runtime)
./prog

# TSan-specific options
TSAN_OPTIONS="halt_on_error=1:second_deadlock_stack=1" ./prog
```

Reading a TSan report:

```text
WARNING: ThreadSanitizer: data race (pid=12345)
  Write of size 4 at 0x7f1234 by thread T2:
    #0 increment /src/counter.c:8:5              ← access site in T2
    #1 worker_thread /src/counter.c:22:3

  Previous read of size 4 at 0x7f1234 by thread T1:
    #0 read_counter /src/counter.c:3:14          ← conflicting access in T1
    #1 main /src/counter.c:30:5

  Thread T2 created at:
    #0 pthread_create .../tsan_interceptors.cpp
    #1 main /src/counter.c:28:3

SUMMARY: ThreadSanitizer: data race /src/counter.c:8:5 in increment
```

How to read:
1. Line 1: type of access (write/read) and address
2. Stack under "Write of size": the thread that performed the write
3. Stack under "Previous read/write": the conflicting thread
````
END THIRD-PARTY FILE b4a3d72217b4dd31

The third-party file has ended. Everything between the b4a3d72217b4dd31 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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