.\"	$NetBSD: localcount.9,v 1.6 2017/06/02 00:32:12 chs Exp $
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.\" by Taylor R. Campbell.
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.Dd June 1, 2017
.Dt LOCALCOUNT 9
.Os
.Sh NAME
.Nm localcount ,
.Nm localcount_init ,
.Nm localcount_fini ,
.Nm localcount_acquire ,
.Nm localcount_release ,
.Nm localcount_drain
.Nd reference-count primitives
.Sh SYNOPSIS
.In sys/localcount.h
.Ft void
.Fn localcount_init "struct localcount *lc"
.Ft void
.Fn localcount_fini "struct localcount *lc"
.Ft void
.Fn localcount_acquire "struct localcount *lc"
.Ft void
.Fn localcount_release "struct localcount *lc" "struct kcondvar *cv" \
"struct kmutex *mtx"
.Ft void
.Fn localcount_drain "struct localcount *lc" "struct kcondvar *cv" \
"struct kmutex *mtx"
.Sh DESCRIPTION
Localcounts are used in the kernel to implement a medium-weight reference
counting mechanism.
During normal operations, localcounts do not need the interprocessor
synchronization associated with
.Xr atomic_ops 3
atomic memory operations, and (unlike
.Xr psref 9 )
.Nm
references can be held across sleeps and can migrate between CPUs.
Draining a
.Nm
requires more expensive interprocessor synchronization than
.Xr atomic_ops 3
(similar to
.Xr psref 9 ) .
And
.Nm
references require eight bytes of memory per object per-CPU, significantly
more than
.Xr atomic_ops 3
and almost always more than
.Xr psref 9 .
.Pp
As a rough heuristic,
.Nm
should be used for classes of objects of which there are perhaps a few dozen
instances (such as
.Xr autoconf 9
devices) but not thousands of instances (such as
network flows) and on which there may be a mixture of long-term I/O waits,
such as xyzread for a device xyz(4), and short-term fast operations, such as
.Dv xyzioctl(IOC_READ_A_CPU_REG) .
.Sh FUNCTIONS
.Bl -tag -width 32n
.It Fn localcount_init "lc"
Dynamically initialize localcount
.Ar lc
for use.
.Pp
No other operations can be performed on a localcount until it has been
initialized.
.It Fn localcount_fini "lc"
Release resources used by localcount
.Ar lc .
The caller must have already called
.Fn localcount_drain .
The localcount may not be used after
.Fn localcount_fini
has been called until it has been re-initialized by
.Fn localcount_init .
.It Fn localcount_acquire "lc"
Acquire a reference to the localcount
.Ar lc .
.It Fn localcount_release "lc" "cv" "mtx"
Release a reference to the localcount
.Ar lc .
If the localcount is currently being drained, the mutex
.Ar mtx
will be used to synchronize updates to the global reference count (i.e.,
the total across all CPUs).
If the reference count goes to zero,
.Fn localcount_release
will broadcast availability of the condvar
.Ar cv .
.It Fn localcount_drain "lc" "cv" "mtx"
Wait for all references to the localcount
.Ar lc
to be released.
The caller must hold the mutex
.Ar mtx ;
the mutex will be released during inter-CPU cross-calls (see
.Xr xcall 9 )
and while waiting on the condvar
.Ar cv .
The same
.Ar cv
and
.Ar mtx
must be used with
.Fn localcount_release .
.Pp
The caller must guarantee that no new references can be acquired with
.Fn localcount_acquire
before calling
.Fn localcount_drain .
For example, any object that may be found in a list and acquired must be
removed from the list before calling
.Fn localcount_drain .
Once the localcount object
.Ar lc
is passed to
.Fn localcount_drain ,
it must be passed to
.Fn localcount_fini
before any other re-use.
.El
.Sh CODE REFERENCES
The core of the localcount implementation is located in
.Pa sys/kern/subr_localcount.c .
.Pp
The header file
.Pa sys/sys/localcount.h
describes the public interface, and interfaces that machine-dependent
code must provide to support localcounts.
.Sh SEE ALSO
.Xr atomic_ops 3 ,
.Xr condvar 9 ,
.Xr mutex 9 ,
.Xr psref 9
.Sh HISTORY
The localcount primitives first appeared in
.Nx 8.0 .
.Sh AUTHORS
.Nm
was designed by
.An -nosplit
.An Taylor R. Campbell ,
who also provided a draft implementation.
The implementation was completed, tested, and integrated by
.An Paul Goyette .
.Sh CAVEATS
The
.Nm
facility does not provide any way to examine the reference count without
actually waiting for the count to reach zero.
.Pp
Waiting for a
.Nm
reference count to drain (reach zero) is a one-shot operation.
Once the
.Nm
has been drained, no further operations are allowed until the
.Nm
has been re-initialized.