Synchronization¶

Overview¶

Synchronization mechanisms coordinate concurrent execution and protect shared resources in multi-threaded and multi-core systems.

Linux provides multiple synchronization primitives designed for different workloads and performance requirements.

This topic covers traditional locking mechanisms, lockless read techniques, and object lifetime management.

Learning Path¶

Userspace Synchronization¶

Locking Primitives - mutexes, condition variables, futexes, and reader-writer locks

Kernel Synchronization¶

Wait Queue - event-driven kernel thread synchronization
Completion - one-shot operation synchronization
Notifier Chain - kernel event broadcast and callback framework
Seqlock - lockless readers with consistent snapshots
RCU - object publication, grace periods, and lifetime management

Synchronization Design Overview¶

Mechanism	Primary Goal	Reader Cost	Writer Cost	Best For
Mutex	Mutual exclusion	Medium	Medium	Shared mutable state
Condition Variable	Event waiting	N/A	N/A	Producer-consumer patterns
Semaphore	Resource counting	N/A	N/A	Resource coordination
Wait Queue	Condition-based waiting	Low	Low	Event-driven kernel threads
Completion	One-shot synchronization	Low	Low	Initialization and operation completion
Notifier Chain	Event propagation	Low	Low	One-to-many event notification
RWLock	Read-heavy workloads	Medium	High	Frequent reads
Seqlock	Consistent snapshot	Very Low	Medium	Statistics and timekeeping
RCU	Object lifetime management	Very Low	High	Read-mostly object access

Choosing the Right Primitive¶

Use Mutex When¶

Only one thread may access shared data at a time.

Examples:

Device state
Configuration updates
Linked list modifications

Use Condition Variable When¶

One thread must wait for another thread
to change system state.

Examples:

Producer-consumer queue
Worker thread notification

Use Wait Queue When¶

Threads must wait until a condition becomes true.

Examples:

Kernel state machines
Worker threads
Producer-consumer pipelines
Event-driven processing

Use Completion When¶

One execution context waits for another
operation to finish.

Examples:

Hardware initialization
Firmware loading
DMA completion
Worker startup synchronization

Use Notifier Chain When¶

One subsystem must notify multiple
independent subscribers about an event.

Examples:

CPU frequency changes
Power management events
Reboot notifications
Network device state changes

Use RWLock When¶

Reads are much more frequent than writes.

Examples:

Configuration tables
Lookup tables

Use Seqlock When¶

Readers require a consistent snapshot
and occasional retries are acceptable.

Examples:

Statistics
Timekeeping
System state snapshots

Use RCU When¶

Readers frequently access shared objects
and writers occasionally replace them.

Examples:

Routing tables
Device lookup tables
Kernel object registries

Relationship Between Topics¶

The synchronization mechanisms covered in this handbook build progressively:

Mutex
    ↓
Condition Variable
    ↓
Wait Queue
    ↓
Completion
    ↓
Notifier Chain
    ↓
RWLock
    ↓
Seqlock
    ↓
RCU

As the progression advances:

More Reader Scalability
        ↓
Less Reader Overhead
        ↓
More Complex Writer Logic

Summary¶

Synchronization is not a single technique but a collection of mechanisms designed for different concurrency problems.

The key design skill is selecting the synchronization primitive that best matches the workload, data access pattern, and performance requirements of the system.