`ps_pgrp' is protected by both kernel lock and `ps_mtx' mutex, so only
one is enough for read only access. `pg_id' has the type of pid_t.

The corresponding enternewpgrp() sets `ps_pgrp' to the initialized new
one with both locks held.

Index: sys/kern/kern_prot.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_prot.c,v
diff -u -p -r1.84 kern_prot.c
--- sys/kern/kern_prot.c	3 Dec 2025 17:05:53 -0000	1.84
+++ sys/kern/kern_prot.c	8 Jun 2026 13:13:21 -0000
@@ -94,7 +94,9 @@ int
 sys_getpgrp(struct proc *p, void *v, register_t *retval)
 {
 
+	mtx_enter(&p->p_p->ps_mtx);
 	*retval = p->p_p->ps_pgrp->pg_id;
+	mtx_leave(&p->p_p->ps_mtx);
 	return (0);
 }
 
Index: sys/kern/syscalls.master
===================================================================
RCS file: /cvs/src/sys/kern/syscalls.master,v
diff -u -p -r1.272 syscalls.master
--- sys/kern/syscalls.master	2 Jun 2026 21:46:21 -0000	1.272
+++ sys/kern/syscalls.master	8 Jun 2026 13:13:21 -0000
@@ -183,7 +183,7 @@
 			    gid_t *gidset); }
 80	STD		{ int sys_setgroups(int gidsetsize, \
 			    const gid_t *gidset); }
-81	STD		{ int sys_getpgrp(void); }
+81	STD NOLOCK	{ int sys_getpgrp(void); }
 82	STD		{ int sys_setpgid(pid_t pid, pid_t pgid); }
 83	STD NOLOCK	{ int sys_futex(uint32_t *f, int op, int val, \
 			    const struct timespec *timeout, uint32_t *g); }