exit: get rid of PROC_SLOCK when checking a process to report, take #2

The suspension counter needs synchronisation through slock, but we don't
need it to check if inspecting the counter is necessary to begin with.
In the common case it is not, thus avoid the lock if possible.

Reviewed by:	kib
Tested by:	pho

Author: mjguzik

sys/kern/kern_exit.c

@@ -1174,6 +1174,7 @@ kern_wait6(struct thread *td, idtype_t idtype, id_t id, int *status,
 	struct proc *p, *q;
 	pid_t pid;
 	int error, nfound, ret;
+	bool report;
 
 	AUDIT_ARG_VALUE((int)idtype);	/* XXX - This is likely wrong! */
 	AUDIT_ARG_PID((pid_t)id);	/* XXX - This may be wrong! */
@@ -1225,36 +1226,38 @@ kern_wait6(struct thread *td, idtype_t idtype, id_t id, int *status,
 		nfound++;
 		PROC_LOCK_ASSERT(p, MA_OWNED);
 
-		if ((options & (WTRAPPED | WUNTRACED)) != 0)
-			PROC_SLOCK(p);
-
 		if ((options & WTRAPPED) != 0 &&
-		    (p->p_flag & P_TRACED) != 0 &&
-		    (p->p_flag & (P_STOPPED_TRACE | P_STOPPED_SIG)) != 0 &&
-		    p->p_suspcount == p->p_numthreads &&
-		    (p->p_flag & P_WAITED) == 0) {
+		    (p->p_flag & P_TRACED) != 0) {
+			PROC_SLOCK(p);
+			report =
+			    ((p->p_flag & (P_STOPPED_TRACE | P_STOPPED_SIG)) &&
+			    p->p_suspcount == p->p_numthreads &&
+			    (p->p_flag & P_WAITED) == 0);
 			PROC_SUNLOCK(p);
+			if (report) {
 			CTR4(KTR_PTRACE,
 			    "wait: returning trapped pid %d status %#x "
 			    "(xstat %d) xthread %d",
 			    p->p_pid, W_STOPCODE(p->p_xsig), p->p_xsig,
 			    p->p_xthread != NULL ?
 			    p->p_xthread->td_tid : -1);
-			report_alive_proc(td, p, siginfo, status, options,
-			    CLD_TRAPPED);
-			return (0);
+				report_alive_proc(td, p, siginfo, status,
+				    options, CLD_TRAPPED);
+				return (0);
+			}
 		}
 		if ((options & WUNTRACED) != 0 &&
-		    (p->p_flag & P_STOPPED_SIG) != 0 &&
-		    p->p_suspcount == p->p_numthreads &&
-		    (p->p_flag & P_WAITED) == 0) {
+		    (p->p_flag & P_STOPPED_SIG) != 0) {
+			PROC_SLOCK(p);
+			report = (p->p_suspcount == p->p_numthreads &&
+			    ((p->p_flag & P_WAITED) == 0));
 			PROC_SUNLOCK(p);
-			report_alive_proc(td, p, siginfo, status, options,
-			    CLD_STOPPED);
-			return (0);
+			if (report) {
+				report_alive_proc(td, p, siginfo, status,
+				    options, CLD_STOPPED);
+				return (0);
+			}
 		}
-		if ((options & (WTRAPPED | WUNTRACED)) != 0)
-			PROC_SUNLOCK(p);
 		if ((options & WCONTINUED) != 0 &&
 		    (p->p_flag & P_CONTINUED) != 0) {
 			report_alive_proc(td, p, siginfo, status, options,