LOCKING(9) | Kernel Developer's Manual | LOCKING(9)

# NAME

locking — introduction to kernel synchronization and interrupt control

# DESCRIPTION

The NetBSD kernel provides several synchronization and interrupt control primitives. This man page aims to give an overview of these interfaces and their proper application. Also included are basic kernel thread control primitives and a rough overview of the NetBSD kernel design.

# KERNEL OVERVIEW

The aim of synchronization, threads and interrupt control in the kernel is:

- To control concurrent access to shared resources (critical sections).
- Spawn tasks from an interrupt in the thread context.
- Mask interrupts from threads.
- Scale on multiple CPU system.

There are three types of contexts in the NetBSD kernel:

- Thread context - running processes (represented by `struct proc`) and light-weight processes (represented by `struct lwp`, also known as kernel threads). Code in this context can sleep, block resources and own address-space.
- Software interrupt context - limited by thread context. Code in this context must be processed shortly. These interrupts don't own any address space context. Software interrupts are a way of deferring hardware interrupts to do more expensive processing at a lower interrupt priority.
- Hard interrupt context - Code in this context must be processed as quickly as possible. It is forbidden for a piece of code to sleep or access long-awaited resources here.

The main differences between processes and kernel threads are:

- A single process can own multiple kernel threads (LWPs).
- A process owns address space context to map userland address space.
- Processes are designed for userland executables and kernel threads for in-kernel tasks. The only process running in the kernel-space is `proc0` (called swapper).

# INTERFACES

## Atomic memory operations

The atomic_ops family of functions provide atomic memory operations. There are 7 classes of atomic memory operations available: addition, logical “and”, compare-and-swap, decrement, increment, logical “or”, swap.
See atomic_ops(3).

## Condition variables

Condition variables (CVs) are used in the kernel to synchronize access to resources that are limited (for example, memory) and to wait for pending I/O operations to complete.
See condvar(9).

## Memory access barrier operations

The membar_ops family of functions provide memory access barrier operations necessary for synchronization in multiprocessor execution environments that have relaxed load and store order.
See membar_ops(3).

## Memory barriers

The memory barriers can be used to control the order in which memory accesses occur, and thus the order in which those accesses become visible to other processors. They can be used to implement “lockless” access to data structures where the necessary barrier conditions are well understood.

## Mutual exclusion primitives

Thread-base adaptive mutexes. These are lightweight, exclusive locks that use threads as the focus of synchronization activity. Adaptive mutexes typically behave like spinlocks, but under specific conditions an attempt to acquire an already held adaptive mutex may cause the acquiring thread to sleep. Sleep activity occurs rarely. Busy-waiting is typically more efficient because mutex hold times are most often short. In contrast to pure spinlocks, a thread holding an adaptive mutex may be pre-empted in the kernel, which can allow for reduced latency where soft real-time application are in use on the system.
See mutex(9).

## Restartable atomic sequences

Restartable atomic sequences are user code only sequences which are guaranteed to execute without preemption. This property is assured by checking the set of restartable atomic sequences registered for a process during cpu_switchto(9). If a process is found to have been preempted during a restartable sequence, then its execution is rolled-back to the start of the sequence by resetting its program counter which is saved in its process control block (PCB).
See ras(9).

## Reader / writer lock primitives

Reader / writer locks (RW locks) are used in the kernel to synchronize access to an object among LWPs (lightweight processes) and soft interrupt handlers. In addition to the capabilities provided by mutexes, RW locks distinguish between read (shared) and write (exclusive) access.
See rwlock(9).

## Functions to modify system interrupt priority level

These functions raise and lower the interrupt priority level. They are used by kernel code to block interrupts in critical sections, in order to protect data structures.
See spl(9).

## Machine-independent software interrupt framework

The software interrupt framework is designed to provide a generic software interrupt mechanism which can be used any time a low-priority callback is required. It allows dynamic registration of software interrupts for loadable drivers, protocol stacks, software interrupt prioritization, software interrupt fair queuing and allows machine-dependent optimizations to reduce cost.
See softint(9).

## Functions to raise the system priority level

The splraiseipl function raises the system priority level to the level specified by `icookie`, which should be a value returned by makeiplcookie(9). In general, device drivers should not make use of this interface. To ensure correct synchronization, device drivers should use the condvar(9), mutex(9), and rwlock(9) interfaces.
See splraiseipl(9).

## Passive serialization mechanism

Passive serialization is a reader / writer synchronization mechanism designed for lock-less read operations. The read operations may happen from software interrupt at `IPL_SOFTCLOCK`.
See pserialize(9).

## Passive reference mechanism

Passive references allow CPUs to cheaply acquire and release passive references to a resource, which guarantee the resource will not be destroyed until the reference is released. Acquiring and releasing passive references requires no interprocessor synchronization, except when the resource is pending destruction.
See psref(9).

## Localcount mechanism

Localcounts are used in the kernel to implement a medium-weight reference counting mechanism. During normal operations, localcounts do not need the interprocessor synchronization associated with atomic_ops(3) atomic memory operations, and (unlike psref(9)) localcount references can be held across sleeps and can migrate between CPUs. Draining a localcount requires more expensive interprocessor synchronization than atomic_ops(3) (similar to psref(9)). And localcount references require eight bytes of memory per object per-CPU, significantly more than atomic_ops(3) and almost always more than psref(9).
See localcount(9).

## Simple do-it-in-thread-context framework

The workqueue utility routines are provided to defer work which is needed to be processed in a thread context.
See workqueue(9).

# USAGE

The following table describes in which contexts the use of the NetBSD kernel interfaces are valid. Synchronization primitives which are available in more than one context can be used to protect shared resources between the contexts they overlap.

interface | thread | softirq | hardirq
atomic_ops(3) | yes | yes | yes
condvar(9) | yes | partly | no
membar_ops(3) | yes | yes | yes
mutex(9) | yes | depends | depends
rwlock(9) | yes | yes | no
softint(9) | yes | yes | yes
spl(9) | yes | no | no
splraiseipl(9) | yes | no | no
pserialize(9) | yes | yes | no
psref(9) | yes | yes | no
localcount(9) | yes | yes | no
workqueue(9) | yes | yes | yes

# SEE ALSO

atomic_ops(3), membar_ops(3), condvar(9), mutex(9), ras(9), rwlock(9), softint(9), spl(9), splraiseipl(9), workqueue(9)

# HISTORY

Initial SMP support was introduced in NetBSD 2.0 and was designed with a giant kernel lock. Through NetBSD 4.0, the kernel used spinlocks and a per-CPU interrupt priority level (the spl(9) system). These mechanisms did not lend themselves well to a multiprocessor environment supporting kernel preemption. The use of thread based (lock) synchronization was limited and the available synchronization primitive (lockmgr) was inefficient and slow to execute. NetBSD 5.0 introduced massive performance improvements on multicore hardware by Andrew Doran. This work was sponsored by The NetBSD Foundation.
A locking manual first appeared in NetBSD 8.0 and was inspired by the corresponding locking manuals in FreeBSD and DragonFly.

# AUTHORS

Kamil Rytarowski <kamil@NetBSD.org>.

April 15, 2017 | NetBSD 8.2