'\" t
.\"     Title: synchronize_sched_expedited
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: May 2018
.\"    Manual: Driver Basics
.\"    Source: Kernel Hackers Manual 3.16.56
.\"  Language: English
.\"
.TH "SYNCHRONIZE_SCHED_EX" "9" "May 2018" "Kernel Hackers Manual 3\&.16\&" "Driver Basics"
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.SH "NAME"
synchronize_sched_expedited \- Brute\-force RCU\-sched grace period
.SH "SYNOPSIS"
.HP \w'void\ synchronize_sched_expedited('u
.BI "void synchronize_sched_expedited(" "void" ");"
.SH "ARGUMENTS"
.PP
\fIvoid\fR
.RS 4
no arguments
.RE
.SH "DESCRIPTION"
.PP
.PP
Wait for an RCU\-sched grace period to elapse, but use a
\(lqbig hammer\(rq
approach to force the grace period to end quickly\&. This consumes significant time on all CPUs and is unfriendly to real\-time workloads, so is thus not recommended for any sort of common\-case code\&. In fact, if you are using
\fBsynchronize_sched_expedited\fR
in a loop, please restructure your code to batch your updates, and then use a single
\fBsynchronize_sched\fR
instead\&.
.PP
Note that it is illegal to call this function while holding any lock that is acquired by a CPU\-hotplug notifier\&. And yes, it is also illegal to call this function from a CPU\-hotplug notifier\&. Failing to observe these restriction will result in deadlock\&.
.PP
This implementation can be thought of as an application of ticket locking to RCU, with sync_sched_expedited_started and sync_sched_expedited_done taking on the roles of the halves of the ticket\-lock word\&. Each task atomically increments sync_sched_expedited_started upon entry, snapshotting the old value, then attempts to stop all the CPUs\&. If this succeeds, then each CPU will have executed a context switch, resulting in an RCU\-sched grace period\&. We are then done, so we use
\fBatomic_cmpxchg\fR
to update sync_sched_expedited_done to match our snapshot \-\- but only if someone else has not already advanced past our snapshot\&.
.PP
On the other hand, if
\fBtry_stop_cpus\fR
fails, we check the value of sync_sched_expedited_done\&. If it has advanced past our initial snapshot, then someone else must have forced a grace period some time after we took our snapshot\&. In this case, our work is done for us, and we can simply return\&. Otherwise, we try again, but keep our initial snapshot for purposes of checking for someone doing our work for us\&.
.PP
If we fail too many times in a row, we fall back to
\fBsynchronize_sched\fR\&.
.SH "COPYRIGHT"
.br