Day61 - Futex, Mutex Internals and Condition Variable Internals

Date

2026-05-27

Goal

Understand how Linux userspace synchronization primitives are implemented.

Focus areas:

• Futex fundamentals
• pthread mutex internals
• Condition variable internals
• Userspace fast path vs kernel slow path
• Relationship between condition state and notification

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Learning Summary

Today focused on understanding the synchronization stack beneath pthread.

The goal was not API usage but understanding how synchronization behaves internally.

Conceptually:

pthread_mutex
=
atomic
+
futex

pthread_cond
=
condition state
+
mutex
+
futex

---

Lab 1 — Observe pthread mutex fast path

Implemented:

• uncontended mutex loop
• strace verification

Observation:

pthread_mutex_lock()

↓

CAS success

↓

userspace only

Result:

• no futex syscall observed
• lock/unlock path remained in userspace

Conclusion:

Linux mutex avoids syscall overhead in uncontended cases.

---

Lab 2 — Implement mini futex mutex

Implemented:

• atomic compare exchange
• futex wait
• futex wake

Architecture:

lock
↓

atomic CAS

↓

futex wait

↓

wake

Observation:

Mutex itself is not implemented by futex.

Futex only provides sleeping and wakeup capability.

Conclusion:

Futex is a synchronization primitive, not a lock.

---

Lab 3 — Understand futex expected semantics

Experiment:

Producer wakes before consumer enters wait.

Observed:

FUTEX_WAKE → 0

consumer later calls wait

↓

EAGAIN

Key learning:

wake may be missed

but state must not

Conclusion:

FUTEX_WAIT(expected) validates state before sleeping.

---

Lab 4 — Build a minimal condition variable

Implemented:

• ready as condition state
• seq as notification sequence
• mutex for coordination
• futex for waiting

Structure:

ready
+
seq
+
mutex
+
futex

Key insight:

Condition state and notification state are independent concepts.

Condition variable does not store events.

It only coordinates waiting.

---

Important Concepts

Mutex

atomic
+
futex

Condition Variable

condition
+
wait queue
+
mutex coordination

Futex

sleep if value == expected
otherwise return EAGAIN

---

Reflection

The most difficult concept today was understanding:

condition
≠
notification

Using ready and seq separately helped reveal how pthread condition variables operate internally.

Another important realization:

Most pthread synchronization objects are userspace state.

Kernel participation only happens during contention.

---

Files Produced

Implemented:

• mutex_fast_path.c
• mutex_contention.c
• mini_futex_mutex.c
• broken_cond_wait.c
• test_my_cond.c

Helper:

• futex_wait()
• futex_wake_one()

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Next

Potential directions:

• pthread_rwlock internals
• glibc mutex internals
• eventfd and synchronization
• wait queue and scheduler internals

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