Index: head/sys/amd64/amd64/machdep.c
===================================================================
--- head/sys/amd64/amd64/machdep.c
+++ head/sys/amd64/amd64/machdep.c
@@ -1978,7 +1978,12 @@
 int
 ptrace_single_step(struct thread *td)
 {
-	td->td_frame->tf_rflags |= PSL_T;
+
+	PROC_LOCK_ASSERT(td->td_proc, MA_OWNED);
+	if ((td->td_frame->tf_rflags & PSL_T) == 0) {
+		td->td_frame->tf_rflags |= PSL_T;
+		td->td_dbgflags |= TDB_STEP;
+	}
 	return (0);
 }
 
@@ -1986,7 +1991,9 @@
 ptrace_clear_single_step(struct thread *td)
 {
 
+	PROC_LOCK_ASSERT(td->td_proc, MA_OWNED);
 	td->td_frame->tf_rflags &= ~PSL_T;
+	td->td_dbgflags &= ~TDB_STEP;
 	return (0);
 }
 
Index: head/sys/amd64/amd64/trap.c
===================================================================
--- head/sys/amd64/amd64/trap.c
+++ head/sys/amd64/amd64/trap.c
@@ -285,8 +285,14 @@
 			signo = SIGTRAP;
 			ucode = TRAP_TRACE;
 			dr6 = rdr6();
-			if (dr6 & DBREG_DR6_BS)
-				frame->tf_rflags &= ~PSL_T;
+			if ((dr6 & DBREG_DR6_BS) != 0) {
+				PROC_LOCK(td->td_proc);
+				if ((td->td_dbgflags & TDB_STEP) != 0) {
+					td->td_frame->tf_rflags &= ~PSL_T;
+					td->td_dbgflags &= ~TDB_STEP;
+				}
+				PROC_UNLOCK(td->td_proc);
+			}
 			break;
 
 		case T_ARITHTRAP:	/* arithmetic trap */
Index: head/sys/i386/i386/machdep.c
===================================================================
--- head/sys/i386/i386/machdep.c
+++ head/sys/i386/i386/machdep.c
@@ -2764,7 +2764,12 @@
 int
 ptrace_single_step(struct thread *td)
 {
-	td->td_frame->tf_eflags |= PSL_T;
+
+	PROC_LOCK_ASSERT(td->td_proc, MA_OWNED);
+	if ((td->td_frame->tf_eflags & PSL_T) == 0) {
+		td->td_frame->tf_eflags |= PSL_T;
+		td->td_dbgflags |= TDB_STEP;
+	}
 	return (0);
 }
 
@@ -2772,7 +2777,9 @@
 ptrace_clear_single_step(struct thread *td)
 {
 
+	PROC_LOCK_ASSERT(td->td_proc, MA_OWNED);
 	td->td_frame->tf_eflags &= ~PSL_T;
+	td->td_dbgflags &= ~TDB_STEP;
 	return (0);
 }
 
Index: head/sys/i386/i386/trap.c
===================================================================
--- head/sys/i386/i386/trap.c
+++ head/sys/i386/i386/trap.c
@@ -337,8 +337,14 @@
 			signo = SIGTRAP;
 			ucode = TRAP_TRACE;
 			dr6 = rdr6();
-			if (dr6 & DBREG_DR6_BS)
-				frame->tf_eflags &= ~PSL_T;
+			if ((dr6 & DBREG_DR6_BS) != 0) {
+				PROC_LOCK(td->td_proc);
+				if ((td->td_dbgflags & TDB_STEP) != 0) {
+					td->td_frame->tf_eflags &= ~PSL_T;
+					td->td_dbgflags &= ~TDB_STEP;
+				}
+				PROC_UNLOCK(td->td_proc);
+			}
 			break;
 
 		case T_ARITHTRAP:	/* arithmetic trap */
Index: head/sys/sys/proc.h
===================================================================
--- head/sys/sys/proc.h
+++ head/sys/sys/proc.h
@@ -465,6 +465,7 @@
 #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 */
+#define	TDB_STEP	0x00002000 /* (x86) PSL_T set for PT_STEP */
 
 /*
  * "Private" flags kept in td_pflags: