Add refcnt_shared() and refcnt_read()

refcnt_shared() checks whether the object has multiple references.
When refcnt_shared() returns zero, the caller is the only reference
holder.

refcnt_read() returns a snapshot of the counter value.

refcnt_shared() suggested by dlg@.

OK dlg@ mvs@

3 files changed, 44 insertions, 5 deletions

diff --git a/share/man/man9/refcnt_init.9 b/share/man/man9/refcnt_init.9
index 1b19d3b653a..cd19b150e93 100644
--- a/share/man/man9/refcnt_init.9
+++ b/share/man/man9/refcnt_init.9
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: refcnt_init.9,v 1.1 2015/09/11 19:13:22 dlg Exp $
+.\"	$OpenBSD: refcnt_init.9,v 1.2 2022/03/16 14:13:01 visa Exp $
 .\"
 .\" Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
 .\"
@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: September 11 2015 $
+.Dd $Mdocdate: March 16 2022 $
 .Dt REFCNT_INIT 9
 .Os
 .Sh NAME
@@ -23,6 +23,8 @@
 .Nm refcnt_rele ,
 .Nm refcnt_rele_wake ,
 .Nm refcnt_finalize ,
+.Nm refcnt_shared ,
+.Nm refcnt_read ,
 .Nm REFCNT_INITIALIZER
 .Nd reference count API
 .Sh SYNOPSIS
@@ -37,6 +39,10 @@
 .Fn "refcnt_rele_wake" "struct refcnt *r"
 .Ft void
 .Fn "refcnt_finalize" "struct refcnt *r" "const char *wmesg"
+.Ft int
+.Fn "refcnt_shared" "struct refcnt *r"
+.Ft unsigned int
+.Fn "refcnt_read" "struct refcnt *r"
 .Fn "REFCNT_INITIALIZER"
 .Sh DESCRIPTION
 The refcnt API provides simple reference counters that can be used
@@ -68,14 +74,26 @@ There may only be one caller to
 per refcnt
 .Fa r .
 .Pp
+.Fn refcnt_shared
+tests if the object has multiple references.
+.Pp
+.Fn refcnt_read
+returns a snapshot of the counter value.
+Its use is discouraged,
+code should use
+.Fn refcnt_shared
+whenever possible.
+.Pp
 .Fn REFCNT_INITIALIZER
 initialises a declaration of a refcnt to 1.
 .Sh CONTEXT
 .Fn refcnt_init ,
 .Fn refcnt_take ,
 .Fn refcnt_rele ,
+.Fn refcnt_rele_wake ,
+.Fn refcnt_shared
 and
-.Fn refcnt_rele_wake
+.Fn refcnt_read
 can be called during autoconf, from process context, or from interrupt
 context.
 .Pp
@@ -85,3 +103,10 @@ can be called from process context.
 .Fn refcnt_rele
 returns a non-zero value if the last reference has been released,
 otherwise 0.
+.Pp
+.Fn refcnt_shared
+returns a non-zero value if the object has multiple references,
+otherwise 0.
+.Pp
+.Fn refcnt_read
+returns a snapshot of the counter value.
diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index 27e36501ae9..e877acff824 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_synch.c,v 1.183 2022/03/10 15:21:08 bluhm Exp $	*/
+/*	$OpenBSD: kern_synch.c,v 1.184 2022/03/16 14:13:01 visa Exp $	*/
 /*	$NetBSD: kern_synch.c,v 1.37 1996/04/22 01:38:37 christos Exp $	*/
 
 /*
@@ -852,6 +852,18 @@ refcnt_finalize(struct refcnt *r, const char *wmesg)
 	}
 }
 
+int
+refcnt_shared(struct refcnt *r)
+{
+	return (atomic_load_int(&r->r_refs) > 1);
+}
+
+unsigned int
+refcnt_read(struct refcnt *r)
+{
+	return (atomic_load_int(&r->r_refs));
+}
+
 void
 cond_init(struct cond *c)
 {
diff --git a/sys/sys/refcnt.h b/sys/sys/refcnt.h
index 26423346e51..7f7b4e87d60 100644
--- a/sys/sys/refcnt.h
+++ b/sys/sys/refcnt.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: refcnt.h,v 1.5 2022/03/10 15:21:08 bluhm Exp $ */
+/*	$OpenBSD: refcnt.h,v 1.6 2022/03/16 14:13:01 visa Exp $ */
 
 /*
  * Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
@@ -37,6 +37,8 @@ void	refcnt_take(struct refcnt *);
 int	refcnt_rele(struct refcnt *);
 void	refcnt_rele_wake(struct refcnt *);
 void	refcnt_finalize(struct refcnt *, const char *);
+int	refcnt_shared(struct refcnt *);
+unsigned int	refcnt_read(struct refcnt *);
 
 #endif /* _KERNEL */