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.\" $NetBSD: pserialize.9,v 1.15 2025/10/01 12:33:10 riastradh Exp $
.\"
.\" Copyright (c) 2011 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd January 26, 2016
.Dt PSERIALIZE 9
.Os
.Sh NAME
.Nm pserialize
.Nd passive serialization mechanism
.Sh SYNOPSIS
.In sys/pserialize.h
.Ft pserialize_t
.Fn pserialize_create "void"
.Ft void
.Fn pserialize_destroy "pserialize_t psz"
.Ft int
.Fn pserialize_read_enter "void"
.Ft void
.Fn pserialize_read_exit "int s"
.Ft void
.Fn pserialize_perform "pserialize_t psz"
.\" -----
.Sh DESCRIPTION
Passive serialization is a reader / writer synchronisation mechanism
designed for lock-less read operations.
The read operations may happen from software interrupt at
.Dv IPL_SOFTCLOCK .
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn pserialize_create
Allocate a new synchronisation object.
.It Fn pserialize_destroy
Destroy the synchronisation object.
No synchronisation activity should happen at this point.
.It Fn pserialize_read_enter
Enter the critical path of the reader side.
Returns an IPL value, which must be passed to
.Xr pserialize_read_exit 9 .
Protected code path is not allowed to block.
.It Fn pserialize_read_exit
Exit the critical path of the reader side.
Takes the IPL value returned by
.Xr pserialize_read_enter 9 .
.It Fn pserialize_perform
Perform the passive serialization on the writer side.
Passing of this function ensures that no readers are in action.
Writers are typically additionally serialized with a separate
mechanism, e.g.
.Xr mutex 9 ,
to remove objects used by readers from a published list.
Operation blocks and it may only be performed from thread context.
.El
.\" -----
.Sh EXAMPLES
Given a global database of frotz records:
.Bd -literal
struct frotz {
...
struct frotz *f_next;
};
static struct {
kmutex_t lock;
pserialize_t psz;
struct frotz *first;
} frobbotzim __cacheline_aligned;
.Ed
.Pp
Create a frotz and publish it, as a writer:
.Bd -literal
struct frotz *f = pool_get(&frotz_pool, PR_WAITOK);
/* Initialize f. */
...
mutex_enter(&frobbotzim.lock);
f->f_next = frobbotzim.first;
/*
* Publish the contents of f->f_next before we publish the
* pointer to f in frobbotzim.first.
*/
atomic_store_release(&frobbotzim.first, f);
mutex_exit(&frobbotzim.lock);
.Ed
.Pp
Find a frotz, as a reader:
.Bd -literal
struct frotz *f;
int error = ENOENT;
int s;
s = pserialize_read_enter();
/* Fetch frobbotzim.first before we fetch anything it point to. */
for (f = atomic_load_consume(&frobbotzim.first);
f != NULL;
f = f->f_next) {
if (f->f_... == key) {
/*
* Grab whatever part of the frotz we need.
* Note that we can't use the frotz after
* pserialize_read_exit, without a stronger
* kind of reference, say a reference count
* managed by atomic_ops(3).
*/
*resultp = f->f_...;
error = 0;
break;
}
}
pserialize_read_exit(s);
return error;
.Ed
.Pp
Remove a frotz, as a writer, and free it once there are no more
readers:
.Bd -literal
struct frotz **fp, *f;
mutex_enter(&frobbotzim.lock);
for (fp = &frobbotzim.first; (f = *fp) != NULL; fp = &f->f_next) {
if (f->f_... == key) {
/*
* Unhook it from the list. Readers may still
* be traversing the list at this point, so
* the next pointer must remain valid and
* memory must remain allocated.
*/
*fp = f->f_next;
break;
}
}
mutex_exit(&frobbotzim.lock);
/*
* Wait for all existing readers to complete. New readers will
* not see f because the list no longer points to it.
*/
pserialize_perform(frobbotzim.psz);
/* Now nobody else can be touching f, so it is safe to free. */
if (f != NULL)
pool_put(&frotz_pool, f);
.Ed
.\" -----
.Sh CODE REFERENCES
The
.Nm
is implemented within the file
.Pa sys/kern/subr_pserialize.c .
.Sh SEE ALSO
.Xr membar_ops 3 ,
.Xr condvar 9 ,
.Xr mutex 9 ,
.Xr rwlock 9
.Rs
.%A Hennessy, et al.
.%T "Passive serialization in a multitasking environment"
.%I US Patent and Trademark Office
.%D February 28, 1989
.%N US Patent 4809168
.Re
.Sh HISTORY
Passive serialization mechanism first appeared in
.Nx 6.0 .
|
