Day 62 - pthread RWLock Internals and Fairness¶

Date¶

2026-05-28

Learn reader-writer synchronization and understand how pthread_rwlock works internally.

Focus areas:

pthread_rwlock allows:

Compared with mutex:

Implemented two fairness policies.

Readers may still acquire lock while writer is waiting.

Result:

Observed writer wait time exceeding 40 seconds.

New readers blocked once writer enters waiting state.

Result:

Important observation:

Whether waiting writers block new readers
defines rwlock fairness policy.

Implemented a simplified spin rwlock using:

Writer states:

WRITER_FREE
WRITER_WAIT_QUEUE
WRITER_ACQUIRED

Implemented reader re-check logic to prevent:

reader and writer entering critical section simultaneously

Flow:

reader_count++
re-check writer state
rollback if writer acquired concurrently

Used:

strace -f -e futex ./rwlock_demo

Observed:

Confirmed rwlock slow path uses futex wait/wake operations.

Started exploring:

Understanding memory visibility is important for synchronization correctness.

Possible next topics:

Planned next topic:

Day63 - Seqlock and RCU Motivation