defer/rcu.tex

% rcu.tex
% SPDX-License-Identifier: CC-BY-SA-3.0

\section{Read-Copy Update (RCU)}
\label{sec:defer:Read-Copy Update (RCU)}
%
\epigraph{``Free'' is a \emph{very} good price!}{\emph{Tom Peterson}}

All of the mechanisms discussed in the preceding sections
used one of a number of approaches to defer specific actions
until they may be carried out safely.
The reference counters discussed in
Section~\ref{sec:defer:Reference Counting}
use explicit counters to defer actions that could disturb readers,
which results in read-side contention and thus poor scalability.
The hazard pointers covered by
Section~\ref{sec:defer:Hazard Pointers}
uses implicit counters in the guise of per-thread lists of pointer.
This avoids read-side contention, but requires readers do stores and
conditional branches, as well as either full memory barriers in read-side
primitives or real-time-unfriendly inter-processor interrupts in
update-side primitives.
The sequence lock presented in
Section~\ref{sec:defer:Sequence Locks}
also avoids read-side contention, but does not protect pointer
traversals and, like hazard pointers, requires either full memory barriers
in read-side primitives, or inter-processor interrupts in update-side
primitives.
These schemes' shortcomings raise the question of
whether it is possible to do better.

This section introduces \emph{read-copy update} (RCU), which provides
an API that allows delays to be identified in the source code,
rather than as expensive updates to shared data.
The remainder of this
section examines RCU from a number of different perspectives.
Section~\ref{sec:defer:Introduction to RCU} provides the classic
introduction to RCU,
Section~\ref{sec:defer:RCU Fundamentals} covers fundamental RCU
concepts,
Section~\ref{sec:defer:RCU Usage} introduces some common uses of RCU,
Section~\ref{sec:defer:RCU Linux-Kernel API} presents the Linux-kernel
API,
Section~\ref{sec:defer:RCU Related Work} covers recent work related
to RCU,
and finally
Section~\ref{sec:defer:RCU Exercises} provides some RCU exercises.

\input{defer/rcuintro}
\input{defer/rcufundamental}
\input{defer/rcuapi}
\input{defer/rcuusage}
\input{defer/rcurelated}