diff --git a/lib/libc/sys/ptrace.2 b/lib/libc/sys/ptrace.2
index 63d412d48477..4f129a8386df 100644
--- a/lib/libc/sys/ptrace.2
+++ b/lib/libc/sys/ptrace.2
@@ -1,7 +1,7 @@
-.\"	$NetBSD: ptrace.2,v 1.31 2010/03/22 19:30:55 joerg Exp $
+.\"	$NetBSD: ptrace.2,v 1.32 2010/04/06 13:50:22 christos Exp $
 .\"
 .\" This file is in the public domain.
-.Dd March 12, 2007
+.Dd April 6, 2010
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -149,7 +149,9 @@ new value for the program counter), or
 to indicate that execution is to pick up where it left off.
 .Fa data
 provides a signal number to be delivered to the traced process as it
-resumes execution, or 0 if no signal is to be sent.
+resumes execution, or 0 if no signal is to be sent.  If a negative
+value is supplied, that is the negative of the LWP ID of the thread to
+be resumed, and only that thread executes.
 .It Dv PT_KILL
 The traced process terminates, as if
 .Dv PT_CONTINUE
@@ -256,8 +258,8 @@ This
 call currently does not stop the child process so it can generate
 inconsistent data.
 .It Dv PT_LWPINFO
-Returns information about the specific thread from the process specified
-in the
+Returns information about a thread from the list of threads for the
+process specified in the
 .Fa pid
 argument.
 The
@@ -274,8 +276,15 @@ struct ptrace_lwpinfo {
 .Pp
 where
 .Fa pl_lwpid
-contains the thread for which to get info.
+contains a thread LWP ID.  Information is returned for the thread
+following the one with the specified ID in the process thread list,
+or for the first thread if
+.Fa pl_lwpid
+is 0.
 Upon return
+.Fa pl_lwpid
+contains the LWP ID of the thread that was found, or 0 if there is
+no thread after the one whose LWP ID was supplied in the call.
 .Fa pl_event
 contains the event that stopped the thread.
 Possible
@@ -303,6 +312,14 @@ lists which requests exist on a given machine.
 Execution continues as in request PT_CONTINUE; however
 as soon as possible after execution of at least one
 instruction, execution stops again.
+If the
+.Fa data
+argument is greater than 0, it contains the LWP ID of the thread to be
+stepped, and any other threads are continued.  If the 
+.Fa data
+argument is less than zero, it contains the negative of the LWP ID of
+the
+thread to be stepped, and only that thread executes.
 .It Dv PT_GETREGS
 This request reads the traced process' machine registers into the
 .Dq Li "struct reg"
@@ -310,6 +327,10 @@ This request reads the traced process' machine registers into the
 .In machine/reg.h )
 pointed to by
 .Fa addr .
+The 
+.Fa data
+argument contains the LWP ID of the thread whose registers are to
+be read.  If zero is supplied, the first thread of the process is read.
 .It Dv PT_SETREGS
 This request is the converse of
 .Dv PT_GETREGS ;
@@ -319,6 +340,10 @@ it loads the traced process' machine registers from the
 .In machine/reg.h )
 pointed to by
 .Fa addr .
+The 
+.Fa data
+argument contains the LWP ID of the thread whose registers are to
+be written.  If zero is supplied, the first thread of the process is written.
 .It Dv PT_GETFPREGS
 This request reads the traced process' floating-point registers into
 the
@@ -327,6 +352,11 @@ the
 .In machine/reg.h )
 pointed to by
 .Fa addr .
+The 
+.Fa data
+argument contains the LWP ID of the thread whose registers are to
+be read.  If zero is supplied, the first thread of the process is 
+read.
 .It Dv PT_SETFPREGS
 This request is the converse of
 .Dv PT_GETFPREGS ;
@@ -336,6 +366,11 @@ it loads the traced process' floating-point registers from the
 .In machine/reg.h )
 pointed to by
 .Fa addr .
+The 
+.Fa data
+argument contains the LWP ID of the thread whose registers are to
+be written.  If zero is supplied, the first thread of the process is 
+written.
 .\" .It Dv PT_SYSCALL
 .\" This request is like
 .\" .Dv PT_CONTINUE
diff --git a/sys/kern/kern_lwp.c b/sys/kern/kern_lwp.c
index c6bce17a4e7b..0664177ccd0e 100644
--- a/sys/kern/kern_lwp.c
+++ b/sys/kern/kern_lwp.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: kern_lwp.c,v 1.141 2010/03/01 21:10:17 darran Exp $	*/
+/*	$NetBSD: kern_lwp.c,v 1.142 2010/04/06 13:50:22 christos Exp $	*/
 
 /*-
  * Copyright (c) 2001, 2006, 2007, 2008, 2009 The NetBSD Foundation, Inc.
@@ -209,7 +209,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_lwp.c,v 1.141 2010/03/01 21:10:17 darran Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_lwp.c,v 1.142 2010/04/06 13:50:22 christos Exp $");
 
 #include "opt_ddb.h"
 #include "opt_lockdebug.h"
@@ -374,6 +374,44 @@ lwp_continue(struct lwp *l)
 	setrunnable(l);
 }
 
+/*
+ * Restart a stopped LWP.
+ *
+ * Must be called with p_lock held, and the LWP NOT locked.  Will unlock the
+ * LWP before return.
+ */
+void
+lwp_unstop(struct lwp *l)
+{
+	struct proc *p = l->l_proc;
+    
+	KASSERT(mutex_owned(proc_lock));
+	KASSERT(mutex_owned(p->p_lock));
+
+	lwp_lock(l);
+
+	/* If not stopped, then just bail out. */
+	if (l->l_stat != LSSTOP) {
+		lwp_unlock(l);
+		return;
+	}
+
+	p->p_stat = SACTIVE;
+	p->p_sflag &= ~PS_STOPPING;
+
+	if (!p->p_waited)
+		p->p_pptr->p_nstopchild--;
+
+	if (l->l_wchan == NULL) {
+		/* setrunnable() will release the lock. */
+		setrunnable(l);
+	} else {
+		l->l_stat = LSSLEEP;
+		p->p_nrlwps++;
+		lwp_unlock(l);
+	}
+}
+
 /*
  * Wait for an LWP within the current process to exit.  If 'lid' is
  * non-zero, we are waiting for a specific LWP.
@@ -1396,11 +1434,25 @@ lwp_delref(struct lwp *l)
 	struct proc *p = l->l_proc;
 
 	mutex_enter(p->p_lock);
+	lwp_delref2(l);
+	mutex_exit(p->p_lock);
+}
+
+/*
+ * Remove one reference to an LWP.  If this is the last reference,
+ * then we must finalize the LWP's death.  The proc mutex is held
+ * on entry.
+ */
+void
+lwp_delref2(struct lwp *l)
+{
+	struct proc *p = l->l_proc;
+
+	KASSERT(mutex_owned(p->p_lock));
 	KASSERT(l->l_stat != LSZOMB);
 	KASSERT(l->l_refcnt > 0);
 	if (--l->l_refcnt == 0)
 		cv_broadcast(&p->p_lwpcv);
-	mutex_exit(p->p_lock);
 }
 
 /*
diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 97876e2958e7..5a490063b791 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: kern_sig.c,v 1.304 2010/03/03 00:47:31 yamt Exp $	*/
+/*	$NetBSD: kern_sig.c,v 1.305 2010/04/06 13:50:22 christos Exp $	*/
 
 /*-
  * Copyright (c) 2006, 2007, 2008 The NetBSD Foundation, Inc.
@@ -66,7 +66,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_sig.c,v 1.304 2010/03/03 00:47:31 yamt Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_sig.c,v 1.305 2010/04/06 13:50:22 christos Exp $");
 
 #include "opt_ptrace.h"
 #include "opt_compat_sunos.h"
@@ -1725,9 +1725,10 @@ sigchecktrace(void)
 
 	/*
 	 * If we are no longer being traced, or the parent didn't
-	 * give us a signal, look for more signals.
+	 * give us a signal, or we're stopping, look for more signals.
 	 */
-	if ((p->p_slflag & PSL_TRACED) == 0 || p->p_xstat == 0)
+	if ((p->p_slflag & PSL_TRACED) == 0 || p->p_xstat == 0 ||
+	    (p->p_sflag & PS_STOPPING) != 0)
 		return 0;
 
 	/*
diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
index e74b5bb86323..3e0e683e96d6 100644
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: sys_process.c,v 1.153 2009/12/17 01:25:10 rmind Exp $	*/
+/*	$NetBSD: sys_process.c,v 1.154 2010/04/06 13:50:22 christos Exp $	*/
 
 /*-
  * Copyright (c) 2008, 2009 The NetBSD Foundation, Inc.
@@ -118,7 +118,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: sys_process.c,v 1.153 2009/12/17 01:25:10 rmind Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sys_process.c,v 1.154 2010/04/06 13:50:22 christos Exp $");
 
 #include "opt_ptrace.h"
 #include "opt_ktrace.h"
@@ -227,7 +227,7 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 		syscallarg(int) data;
 	} */
 	struct proc *p = l->l_proc;
-	struct lwp *lt;
+	struct lwp *lt, *lt2;
 	struct proc *t;				/* target process */
 	struct uio uio;
 	struct iovec iov;
@@ -235,7 +235,8 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 	struct ptrace_lwpinfo pl;
 	struct vmspace *vm;
 	int error, write, tmp, req, pheld;
-	int signo;
+	int signo = 0;
+	int resume_all;
 	ksiginfo_t ksi;
 	char *path;
 	int len;
@@ -463,6 +464,7 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 	write = 0;
 	*retval = 0;
 	tmp = 0;
+	resume_all = 1;
 
 	switch (req) {
 	case  PT_TRACE_ME:
@@ -596,6 +598,44 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 		}
 		p->p_trace_enabled = trace_is_enabled(p);
 
+		/*
+		 * Pick up the LWPID, if supplied.  There are two cases:
+		 * data < 0 : step or continue single thread, lwp = -data
+		 * data > 0 in PT_STEP : step this thread, continue others
+		 * For operations other than PT_STEP, data > 0 means
+		 * data is the signo to deliver to the process.
+		 */
+		tmp = SCARG(uap, data);
+		if (tmp >= 0) {
+#ifdef PT_STEP
+			if (req == PT_STEP)
+				signo = 0;
+			else
+#endif
+			{
+				signo = tmp;
+				tmp = 0;	/* don't search for LWP */
+			}
+		}
+		else
+			tmp = -tmp;
+		
+		if (tmp > 0) {
+			if (req == PT_DETACH) {
+				error = EINVAL;
+				break;
+			}
+			lwp_delref2 (lt);
+			lt = lwp_find(t, tmp);
+			if (lt == NULL) {
+				error = ESRCH;
+				break;
+			}
+			lwp_addref(lt);
+			resume_all = 0;
+			signo = 0;
+		}
+			
 		/*
 		 * From the 4.4BSD PRM:
 		 * "The data argument is taken as a signal number and the
@@ -609,7 +649,7 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 		 */
 
 		/* Check that the data is a valid signal number or zero. */
-		if (SCARG(uap, data) < 0 || SCARG(uap, data) >= NSIG) {
+		if (signo < 0 || signo >= NSIG) {
 			error = EINVAL;
 			break;
 		}
@@ -623,7 +663,17 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 #ifdef PT_STEP
 		/*
 		 * Arrange for a single-step, if that's requested and possible.
+		 * More precisely, set the single step status as requested for
+		 * the requested thread, and clear it for other threads.
 		 */
+		LIST_FOREACH(lt2, &t->p_lwps, l_sibling) {
+			if (lt != lt2) 
+			{
+				lwp_lock(lt2);
+				process_sstep(lt2, 0);
+				lwp_unlock(lt2);
+			}
+		}
 		error = process_sstep(lt, req == PT_STEP);
 		if (error)
 			break;
@@ -640,8 +690,6 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 			/* not being traced any more */
 			t->p_opptr = NULL;
 		}
-
-		signo = SCARG(uap, data);
 	sendsig:
 		/* Finally, deliver the requested signal (or none). */
 		if (t->p_stat == SSTOP) {
@@ -651,7 +699,10 @@ sys_ptrace(struct lwp *l, const struct sys_ptrace_args *uap, register_t *retval)
 			 * an LWP runs to see it.
 			 */
 			t->p_xstat = signo;
-			proc_unstop(t);
+			if (resume_all)
+				proc_unstop(t);
+			else
+				lwp_unstop(lt);
 		} else if (signo != 0) {
 			KSI_INIT_EMPTY(&ksi);
 			ksi.ksi_signo = signo;
diff --git a/sys/sys/lwp.h b/sys/sys/lwp.h
index df11ebeb7056..ed502f967a12 100644
--- a/sys/sys/lwp.h
+++ b/sys/sys/lwp.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: lwp.h,v 1.128 2010/02/21 02:11:39 darran Exp $	*/
+/*	$NetBSD: lwp.h,v 1.129 2010/04/06 13:50:22 christos Exp $	*/
 
 /*-
  * Copyright (c) 2001, 2006, 2007, 2008, 2009 The NetBSD Foundation, Inc.
@@ -298,6 +298,7 @@ void	lwp_relock(lwp_t *, kmutex_t *);
 int	lwp_trylock(lwp_t *);
 void	lwp_addref(lwp_t *);
 void	lwp_delref(lwp_t *);
+void	lwp_delref2(lwp_t *);
 void	lwp_drainrefs(lwp_t *);
 bool	lwp_alive(lwp_t *);
 lwp_t	*lwp_find_first(proc_t *);
@@ -307,6 +308,7 @@ lwp_t	*lwp_find_first(proc_t *);
 void	lwpinit(void);
 int 	lwp_wait1(lwp_t *, lwpid_t, lwpid_t *, int);
 void	lwp_continue(lwp_t *);
+void	lwp_unstop(lwp_t *);
 void	cpu_setfunc(lwp_t *, void (*)(void *), void *);
 void	startlwp(void *);
 void	upcallret(lwp_t *);