Add lock stack trace saving for witness(4).

This lets witness(4) save a stack trace on each lock acquisition.
The saved traces can be viewed in ddb(4) when showing the currently
held locks, which may help when debugging incorrect locking.

Sample output:

ddb{0}> show all locks
Process 63836 (rm) thread 0xffff8000221e52c8 (435004)
exclusive rrwlock inode r = 0 (0xfffffd8119a092c0) locked @ /usr/src/sys/ufs/ufs/ufs_vnops.c:1547
#0  witness_lock+0x419
#1  _rw_enter+0x2bb
#2  _rrw_enter+0x42
#3  VOP_LOCK+0x3f
#4  vn_lock+0x36
#5  vfs_lookup+0xa1
#6  namei+0x2b3
#7  dounlinkat+0x85
#8  syscall+0x338
#9  Xsyscall+0x128
exclusive kernel_lock &kernel_lock r = 1 (0xffffffff81e6a5f0) locked @ /usr/src/sys/arch/amd64/amd64/intr.c:525
#0  witness_lock+0x419
#1  syscall+0x2b6
#2  Xsyscall+0x128

The saving adds overhead, so it is not enabled by default. It can be
taken into use by setting sysctl kern.witness.locktrace=1 at runtime
or by defining WITNESS_LOCKTRACE in the kernel configuration.

Feedback and OK anton@

2 files changed, 15 insertions, 4 deletions

diff --git a/share/man/man4/options.4 b/share/man/man4/options.4
index ce991e68141..6c9f58b9935 100644
--- a/share/man/man4/options.4
+++ b/share/man/man4/options.4
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: options.4,v 1.261 2019/02/06 10:59:49 visa Exp $
+.\"	$OpenBSD: options.4,v 1.262 2019/02/07 15:11:38 visa Exp $
 .\"	$NetBSD: options.4,v 1.21 1997/06/25 03:13:00 thorpej Exp $
 .\"
 .\" Copyright (c) 1998 Theo de Raadt
@@ -34,7 +34,7 @@
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
 .\"
-.Dd $Mdocdate: February 6 2019 $
+.Dd $Mdocdate: February 7 2019 $
 .Dt OPTIONS 4
 .Os
 .Sh NAME
@@ -141,6 +141,14 @@ See
 Maximum number of lock types that are tracked by
 .Xr witness 4 .
 It defaults to 1536.
+.It Cd option WITNESS_LOCKTRACE
+Enable
+.Xr witness 4
+lock stack trace saving at boot.
+The feature is disabled by default and has to be enabled by setting the
+.Va kern.witness.locktrace
+.Xr sysctl 8
+variable.
 .It Cd option WITNESS_WATCH
 Enable
 .Xr witness 4
diff --git a/share/man/man9/malloc.9 b/share/man/man9/malloc.9
index 0b958b0649b..729b76eec17 100644
--- a/share/man/man9/malloc.9
+++ b/share/man/man9/malloc.9
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: malloc.9,v 1.66 2019/01/18 17:52:18 visa Exp $
+.\"	$OpenBSD: malloc.9,v 1.67 2019/02/07 15:11:38 visa Exp $
 .\"	$NetBSD: malloc.9,v 1.2 1996/10/30 05:29:54 lukem Exp $
 .\"
 .\" Copyright (c) 1996 The NetBSD Foundation, Inc.
@@ -28,7 +28,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: January 18 2019 $
+.Dd $Mdocdate: February 7 2019 $
 .Dt MALLOC 9
 .Os
 .Sh NAME
@@ -267,6 +267,9 @@ USB general.
 USB device driver.
 .It Dv M_USBHC
 USB host controller.
+.It Dv M_WITNESS
+.Xr witness 4
+memory.
 .It Dv M_MEMDESC
 Memory range.
 .It Dv M_CRYPTO_DATA