Day60 - Atomics / Memory Ordering / Futex Foundation

Date

2026-05-26

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Goals

Learn synchronization foundations in Linux userspace.

Topics:

• race condition
• C11 atomics
• compare-and-swap
• memory ordering
• futex concept
• synchronization primitive selection

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Completed

Lab1 — Race Condition

Implemented shared counter without synchronization.

Result:

Expected:

2000000

Observed:

counter=1199812

Finding:

counter++

↓

load

↓

modify

↓

store

Operation is not atomic.

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Lab2 — Atomic Counter

Replaced shared integer with:

atomic_uint
atomic_fetch_add()

Result:

counter=2000000

Finding:

Atomic operation guarantees correctness.

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Lab3 — CAS Spin Lock

Implemented custom spin lock using:

atomic_compare_exchange_strong()

Protected:

counter++

Result:

counter=2000000

Observation:

CPU contention became noticeable.

Learned:

• busy wait
• cache contention
• lock overhead

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Lab4 — Memory Ordering

Implemented producer / consumer synchronization.

Version 1:

bool ready

Version 2:

atomic_load_explicit()

atomic_store_explicit()

memory_order_acquire

memory_order_release

Result:

wrong_count=0

Learned:

Visibility and ordering are independent from atomicity.

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Key Takeaways

Synchronization hierarchy:

READ_ONCE

↓

atomic

↓

CAS

↓

futex

↓

mutex

↓

condition variable

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Next

Day61:

• pthread mutex internals
• futex wait/wake
• condition variable internals