Index: sys/kern/kern_sig.c
===================================================================
--- sys/kern/kern_sig.c
+++ sys/kern/kern_sig.c
@@ -2576,7 +2576,15 @@
 			    p->p_xthread == NULL)) {
 				p->p_xsig = sig;
 				p->p_xthread = td;
-				td->td_dbgflags &= ~TDB_FSTP;
+
+				/*
+				 * If we are on sleepqueue already,
+				 * let sleepqueue code decide if it
+				 * needs to go sleep after attach.
+				 */
+				if (td->td_wchan == NULL)
+					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);
Index: sys/kern/subr_sleepqueue.c
===================================================================
--- sys/kern/subr_sleepqueue.c
+++ sys/kern/subr_sleepqueue.c
@@ -498,6 +498,19 @@
 			} else {
 				mtx_unlock(&ps->ps_mtx);
 			}
+
+			/*
+			 * Do not go into sleep if this thread was the
+			 * ptrace(2) attach leader.  cursig() consumed
+			 * SIGSTOP from PT_ATTACH, but we usually act
+			 * on the signal by interrupting sleep, and
+			 * should do it there as well.
+			 */
+			if ((td->td_dbgflags & TDB_FSTP) != 0) {
+				if (ret == 0)
+					ret = EINTR;
+				td->td_dbgflags &= ~TDB_FSTP;
+			}
 		}
 		/*
 		 * Lock the per-process spinlock prior to dropping the PROC_LOCK