Day61 - Futex, Mutex Internals and Condition Variable Internals

Objective

Understand how Linux userspace synchronization primitives are built.

This lab focuses on:

• pthread mutex fast path
• futex wait and wake behavior
• expected value semantics
• condition variable internals

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Environment

Platform:

Raspberry Pi 5
Debian GNU/Linux
Kernel: 6.x

Tools:

gcc
pthread
strace

Build:

gcc <file>.c -pthread -Wall -Wextra -O2

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Lab 1 — Observe pthread mutex fast path

Source:

mutex_fast_path.c

Run:

strace -e futex ./mutex_fast

Expected:

(no futex syscall)

Observation:

• uncontended lock path remains in userspace
• no kernel scheduling involved

Architecture:

pthread_mutex

↓

atomic

↓

success

↓

return

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Lab 2 — Observe mutex contention

Source:

mutex_contention.c

Run:

strace -f -e futex ./mutex_contention

Expected:

FUTEX_WAIT_PRIVATE
FUTEX_WAKE_PRIVATE

Observed:

lock fail

↓

futex wait

↓

wake

↓

retry

Conclusion:

mutex only enters kernel under contention.

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Lab 3 — Implement mini futex mutex

Source:

mini_futex_mutex.c

Implemented:

futex_wait()
futex_wake_one()
mini_mutex_lock()
mini_mutex_unlock()

Pseudo flow:

lock()

CAS

success → return

fail → futex wait

Unlock:

store unlock

↓

wake

Conclusion:

futex is not a lock.

It only coordinates sleeping.

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Lab 4 — Validate futex expected behavior

Source:

broken_cond_wait.c

Scenario:

producer wake

↓

consumer wait later

Observed:

FUTEX_WAKE = 0

↓

FUTEX_WAIT

↓

EAGAIN

Meaning:

Kernel checks:

*addr == expected

before sleeping.

Conclusion:

wake can be missed.

state must not be missed.

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Lab 5 — Build minimal condition variable

Source:

test_my_cond.c

Implemented:

struct my_cond {
    atomic_int seq;
};

Wait:

snapshot seq

unlock

futex wait

relock

Signal:

seq++

wake

Usage:

while (!ready)
    my_cond_wait()

Producer:

ready = true

my_cond_signal()

Conclusion:

condition state and notification state are separate.

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Comparison

Component	Responsibility
ready	Condition
mutex	Protect state
seq	Notification
futex	Sleep and wake

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Key Takeaways

• mutex fast path stays in userspace
• futex uses expected value validation
• condition variable stores no data
• wait queue and condition are separate concepts

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