.\"	$NetBSD: pserialize.9,v 1.12 2016/01/26 01:09:56 riastradh Exp $
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.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 must be additionally serialized with a separate mechanism,
e.g.
.Xr mutex 9 .
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(\*[Am]frotz_pool, PR_WAITOK);

	/* Initialize f.  */
	...

	mutex_enter(\*[Am]frobbotzim.lock);
	f->f_next = frobbotzim.first;
	/*
	 * Publish the contents of f->f_next before we publish the
	 * pointer to f in frobbotzim.first.
	 */
	membar_producer();
	frobbotzim.first = f;
	mutex_exit(\*[Am]frobbotzim.lock);
.Ed
.Pp
Find a frotz, as a reader:
.Bd -literal
	struct frotz *f;
	int error = ENOENT;
	int s;

	s = pserialize_read_enter();
	for (f = frobbotzim.first; f != NULL; f = f->f_next) {
		/* Fetch f before we fetch anything f points to.  */
		membar_datadep_consumer();
		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(\*[Am]frobbotzim.lock);
	for (fp = \*[Am]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;
		}
	}
	/*
	 * 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.  */
	mutex_exit(\*[Am]frobbotzim.lock);

	if (f != NULL)
		pool_put(\*[Am]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 .