Index: bin/ps/ps.1
===================================================================
--- bin/ps/ps.1
+++ bin/ps/ps.1
@@ -360,6 +360,7 @@
 .It Dv "P2_NOTRACE" Ta No "0x00000002" Ta "No ptrace(2) attach or coredumps"
 .It Dv "P2_NOTRACE_EXEC" Ta No "0x00000004" Ta "Keep P2_NOPTRACE on exec(2)"
 .It Dv "P2_AST_SU" Ta No "0x00000008" Ta "Handles SU ast for kthreads"
+.It Dv "P2_PTRACE_FSTP" Ta No "0x00000010" Ta "SIGSTOP from PT_ATTACH not yet handled"
 .El
 .It Cm label
 The MAC label of the process.
Index: sys/kern/kern_exit.c
===================================================================
--- sys/kern/kern_exit.c
+++ sys/kern/kern_exit.c
@@ -478,7 +478,7 @@
 			q->p_flag &= ~(P_TRACED | P_STOPPED_TRACE);
 			q->p_ptevents = 0;
 			FOREACH_THREAD_IN_PROC(q, tdt)
-				tdt->td_dbgflags &= ~TDB_SUSPEND;
+				tdt->td_dbgflags &= ~(TDB_SUSPEND | TDB_XSIG);
 			kern_psignal(q, SIGKILL);
 		}
 		PROC_UNLOCK(q);
Index: sys/kern/kern_fork.c
===================================================================
--- sys/kern/kern_fork.c
+++ sys/kern/kern_fork.c
@@ -1074,15 +1074,13 @@
 			 * parent's children, do it now.
 			 */
 			dbg = p->p_pptr->p_pptr;
-			p->p_flag |= P_TRACED;
-			p->p_ptevents = PTRACE_DEFAULT;
-			p->p_oppid = p->p_pptr->p_pid;
+			proc_set_traced(p);
 			CTR2(KTR_PTRACE,
 		    "fork_return: attaching to new child pid %d: oppid %d",
 			    p->p_pid, p->p_oppid);
 			proc_reparent(p, dbg);
 			sx_xunlock(&proctree_lock);
-			td->td_dbgflags |= TDB_CHILD | TDB_SCX;
+			td->td_dbgflags |= TDB_CHILD | TDB_SCX | TDB_FSTP;
 			ptracestop(td, SIGSTOP);
 			td->td_dbgflags &= ~(TDB_CHILD | TDB_SCX);
 		} else {
Index: sys/kern/kern_sig.c
===================================================================
--- sys/kern/kern_sig.c
+++ sys/kern/kern_sig.c
@@ -2526,14 +2526,26 @@
 			PROC_SUNLOCK(p);
 			return (sig);
 		}
+
 		/*
-		 * Just make wait() to work, the last stopped thread
-		 * will win.
+		 * Make wait(2) to work.  Ensure that right after the
+		 * attach, the thread which decided to become the
+		 * leader of attach, gets reported to the waiter.
+		 * Otherwise, just avoid overwriting other's thread
+		 * assignment to p_xthread.  If already seeing other
+		 * thread in p_xthread, we will get a chance to report
+		 * itself on next iteration.
 		 */
-		p->p_xsig = sig;
-		p->p_xthread = td;
-		p->p_flag |= (P_STOPPED_SIG|P_STOPPED_TRACE);
-		sig_suspend_threads(td, p, 0);
+		if ((td->td_dbgflags & TDB_FSTP) != 0 ||
+		    ((p->p_flag & P2_PTRACE_FSTP) == 0 &&
+		    p->p_xthread == NULL)) {
+			p->p_xsig = sig;
+			p->p_xthread = td;
+			td->td_dbgflags &= ~TDB_FSTP;
+			p->p_flag2 &= ~P2_PTRACE_FSTP;
+			p->p_flag |= P_STOPPED_SIG | P_STOPPED_TRACE;
+			sig_suspend_threads(td, p, 0);
+		}
 		if ((td->td_dbgflags & TDB_STOPATFORK) != 0) {
 			td->td_dbgflags &= ~TDB_STOPATFORK;
 			cv_broadcast(&p->p_dbgwait);
@@ -2726,7 +2738,20 @@
 			SIG_STOPSIGMASK(sigpending);
 		if (SIGISEMPTY(sigpending))	/* no signal to send */
 			return (0);
-		sig = sig_ffs(&sigpending);
+		if ((p->p_flag & (P_TRACED | P_PPTRACE)) == P_TRACED &&
+		    (p->p_flag2 & P2_PTRACE_FSTP) != 0 &&
+		    SIGISMEMBER(sigpending, SIGSTOP)) {
+			/*
+			 * If debugger just attached, always consume
+			 * SIGSTOP from ptrace(PT_ATTACH) first, to
+			 * execute the debugger attach ritual in
+			 * order.
+			 */
+			sig = SIGSTOP;
+			td->td_dbgflags |= TDB_FSTP;
+		} else {
+			sig = sig_ffs(&sigpending);
+		}
 
 		if (p->p_stops & S_SIG) {
 			mtx_unlock(&ps->ps_mtx);
@@ -2743,7 +2768,7 @@
 			sigqueue_delete(&p->p_sigqueue, sig);
 			continue;
 		}
-		if (p->p_flag & P_TRACED && (p->p_flag & P_PPTRACE) == 0) {
+		if ((p->p_flag & (P_TRACED | P_PPTRACE)) == P_TRACED) {
 			/*
 			 * If traced, always stop.
 			 * Remove old signal from queue before the stop.
@@ -2846,6 +2871,8 @@
 				mtx_unlock(&ps->ps_mtx);
 				WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK,
 				    &p->p_mtx.lock_object, "Catching SIGSTOP");
+				sigqueue_delete(&td->td_sigqueue, sig);
+				sigqueue_delete(&p->p_sigqueue, sig);
 				p->p_flag |= P_STOPPED_SIG;
 				p->p_xsig = sig;
 				PROC_SLOCK(p);
@@ -2853,7 +2880,7 @@
 				thread_suspend_switch(td, p);
 				PROC_SUNLOCK(p);
 				mtx_lock(&ps->ps_mtx);
-				break;
+				goto next;
 			} else if (prop & SA_IGNORE) {
 				/*
 				 * Except for SIGCONT, shouldn't get here.
@@ -2884,6 +2911,7 @@
 		}
 		sigqueue_delete(&td->td_sigqueue, sig);	/* take the signal! */
 		sigqueue_delete(&p->p_sigqueue, sig);
+next:;
 	}
 	/* NOTREACHED */
 }
Index: sys/kern/sys_process.c
===================================================================
--- sys/kern/sys_process.c
+++ sys/kern/sys_process.c
@@ -692,6 +692,17 @@
 #define	PROC_WRITE(w, t, a)	proc_write_ ## w (t, a)
 #endif
 
+void
+proc_set_traced(struct proc *p)
+{
+
+	PROC_LOCK_ASSERT(p, MA_OWNED);
+	p->p_flag |= P_TRACED;
+	p->p_flag2 |= P2_PTRACE_FSTP;
+	p->p_ptevents = PTRACE_DEFAULT;
+	p->p_oppid = p->p_pptr->p_pid;
+}
+
 int
 kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 {
@@ -899,11 +910,9 @@
 	switch (req) {
 	case PT_TRACE_ME:
 		/* set my trace flag and "owner" so it can read/write me */
-		p->p_flag |= P_TRACED;
-		p->p_ptevents = PTRACE_DEFAULT;
+		proc_set_traced(p);
 		if (p->p_flag & P_PPWAIT)
 			p->p_flag |= P_PPTRACE;
-		p->p_oppid = p->p_pptr->p_pid;
 		CTR1(KTR_PTRACE, "PT_TRACE_ME: pid %d", p->p_pid);
 		break;
 
@@ -918,9 +927,7 @@
 		 * The old parent is remembered so we can put things back
 		 * on a "detach".
 		 */
-		p->p_flag |= P_TRACED;
-		p->p_ptevents = PTRACE_DEFAULT;
-		p->p_oppid = p->p_pptr->p_pid;
+		proc_set_traced(p);
 		if (p->p_pptr != td->td_proc) {
 			proc_reparent(p, td->td_proc);
 		}
@@ -1088,6 +1095,17 @@
 				    p->p_pid, data);
 			p->p_oppid = 0;
 			p->p_ptevents = 0;
+			FOREACH_THREAD_IN_PROC(p, td3) {
+				td3->td_dbgflags &= ~(TDB_XSIG | TDB_FSTP);
+				if ((p->p_flag2 & P2_PTRACE_FSTP) != 0) {
+					sigqueue_delete(&td3->td_sigqueue,
+					    SIGSTOP);
+				}
+			}
+			if ((p->p_flag2 & P2_PTRACE_FSTP) != 0) {
+				sigqueue_delete(&p->p_sigqueue, SIGSTOP);
+				p->p_flag2 &= ~P2_PTRACE_FSTP;
+			}
 
 			/* should we send SIGCHLD? */
 			/* childproc_continued(p); */
@@ -1108,7 +1126,7 @@
 
 			if (req == PT_DETACH) {
 				FOREACH_THREAD_IN_PROC(p, td3)
-					td3->td_dbgflags &= ~TDB_SUSPEND; 
+					td3->td_dbgflags &= ~TDB_SUSPEND;
 			}
 			/*
 			 * unsuspend all threads, to not let a thread run,
Index: sys/sys/proc.h
===================================================================
--- sys/sys/proc.h
+++ sys/sys/proc.h
@@ -423,6 +423,7 @@
 #define	TDB_BORN	0x00000200 /* New LWP indicator for ptrace() */
 #define	TDB_EXIT	0x00000400 /* Exiting LWP indicator for ptrace() */
 #define	TDB_VFORK	0x00000800 /* vfork indicator for ptrace() */
+#define	TDB_FSTP	0x00001000 /* The thread is PT_ATTACH leader */
 
 /*
  * "Private" flags kept in td_pflags:
@@ -713,6 +714,7 @@
 #define	P2_NOTRACE	0x00000002	/* No ptrace(2) attach or coredumps. */
 #define	P2_NOTRACE_EXEC 0x00000004	/* Keep P2_NOPTRACE on exec(2). */
 #define	P2_AST_SU	0x00000008	/* Handles SU ast for kthreads. */
+#define	P2_PTRACE_FSTP	0x00000010 /* SIGSTOP from PT_ATTACH not yet handled. */
 
 /* Flags protected by proctree_lock, kept in p_treeflags. */
 #define	P_TREE_ORPHANED		0x00000001	/* Reparented, on orphan list */
@@ -1003,6 +1005,7 @@
 struct proc *proc_realparent(struct proc *child);
 void	proc_reap(struct thread *td, struct proc *p, int *status, int options);
 void	proc_reparent(struct proc *child, struct proc *newparent);
+void	proc_set_traced(struct proc *p);
 struct	pstats *pstats_alloc(void);
 void	pstats_fork(struct pstats *src, struct pstats *dst);
 void	pstats_free(struct pstats *ps);