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Introduce membar_datadep_consumer.

Discussed briefly on tech-kern without objection:

https://mail-index.netbsd.org/tech-kern/2014/11/20/msg018054.html
https://mail-index.netbsd.org/tech-kern/2015/01/07/msg018326.html

.\"	$NetBSD: membar_ops.3,v 1.4 2015/01/08 22:27:17 riastradh Exp $
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.\" by Jason R. Thorpe.
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.Dd November 20, 2014
.Dt MEMBAR_OPS 3
.Os
.Sh NAME
.Nm membar_ops ,
.Nm membar_enter ,
.Nm membar_exit ,
.Nm membar_producer ,
.Nm membar_consumer ,
.Nm membar_sync
.Nd memory access barrier operations
.\" .Sh LIBRARY
.\" .Lb libc
.Sh SYNOPSIS
.In sys/atomic.h
.\"
.Ft void
.Fn membar_enter "void"
.Ft void
.Fn membar_exit "void"
.Ft void
.Fn membar_producer "void"
.Ft void
.Fn membar_consumer "void"
.Ft void
.Fn membar_datadep_consumer "void"
.Ft void
.Fn membar_sync "void"
.Sh DESCRIPTION
The
.Nm membar_ops
family of functions provide memory access barrier operations necessary
for synchronization in multiprocessor execution environments that have
relaxed load and store order.
.Pp
.Bl -tag -width "mem"
.It Fn membar_enter
Any store preceding
.Fn membar_enter
will reach global visibility before all loads and stores following it.
.Pp
.Fn membar_enter
is typically used in code that implements locking primitives to ensure
that a lock protects its data.
.It Fn membar_exit
All loads and stores preceding
.Fn membar_exit
will reach global visibility before any store that follows it.
.Pp
.Fn membar_exit
is typically used in code that implements locking primitives to ensure
that a lock protects its data.
.It Fn membar_producer
All stores preceding the memory barrier will reach global visibility
before any stores after the memory barrier reach global visibility.
.It Fn membar_consumer
All loads preceding the memory barrier will complete before any loads
after the memory barrier complete.
.It Fn membar_datadep_consumer
Same as
.Fn membar_consumer ,
but limited to loads of addresses dependent on prior loads, or
.Sq data-dependent
loads:
.Bd -literal -offset indent
int **pp, *p, v;

p = *pp;
membar_datadep_consumer();
v = *p;
consume(v);
.Ed
.Pp
Does not guarantee ordering of loads in branches, or
.Sq control-dependent
loads -- you must use
.Fn membar_consumer
instead:
.Bd -literal -offset indent
int *ok, *p, v;

if (*ok) {
	membar_consumer();
	v = *p;
	consume(v);
}
.Ed
.Pp
Most CPUs do not reorder data-dependent loads (i.e., most CPUs
guarantee that cached values are not stale in that case), so
.Fn membar_datadep_consumer
is a no-op on those CPUs.
.It Fn membar_sync
All loads and stores preceding the memory barrier will complete and
reach global visibility before any loads and stores after the memory
barrier complete and reach global visibility.
.El
.Sh SEE ALSO
.Xr atomic_ops 3
.Sh HISTORY
The
.Nm membar_ops
functions first appeared in
.Nx 5.0 .
The data-dependent load barrier,
.Fn membar_datadep_consumer ,
first appeared in
.Nx 7.0 .