i386 is stable running dpb(1) on NFS with many fstat(1) loops more
than 4 hours.


> 
> My previous similar diffs were failed in NFS cases, so it makes sense to
> drop non NFS setups.
> 
> Index: sys/kern/kern_exit.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_exit.c,v
> diff -u -p -r1.230 kern_exit.c
> --- sys/kern/kern_exit.c	6 Aug 2024 18:41:20 -0000	1.230
> +++ sys/kern/kern_exit.c	9 Aug 2024 11:51:30 -0000
> @@ -151,6 +151,9 @@ exit1(struct proc *p, int xexit, int xsi
> 			    PS_ISPWAIT);
> 			wakeup(pr->ps_pptr);
> 		}
> +
> +		/* Wait for concurrent `allprocess' loops */
> +		refcnt_finalize(&pr->ps_refcnt, "psdtor");
> 	}
> 
> 	/* unlink ourselves from the active threads */
> Index: sys/kern/kern_fork.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_fork.c,v
> diff -u -p -r1.261 kern_fork.c
> --- sys/kern/kern_fork.c	6 Aug 2024 08:44:54 -0000	1.261
> +++ sys/kern/kern_fork.c	9 Aug 2024 11:51:30 -0000
> @@ -178,6 +178,8 @@ thread_new(struct proc *parent, vaddr_t 
> void
> process_initialize(struct process *pr, struct proc *p)
> {
> +	refcnt_init(&pr->ps_refcnt);
> +
> 	/* initialize the thread links */
> 	pr->ps_mainproc = p;
> 	TAILQ_INIT(&pr->ps_threads);
> Index: sys/kern/kern_sysctl.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_sysctl.c,v
> diff -u -p -r1.436 kern_sysctl.c
> --- sys/kern/kern_sysctl.c	8 Aug 2024 15:02:36 -0000	1.436
> +++ sys/kern/kern_sysctl.c	9 Aug 2024 11:51:30 -0000
> @@ -1686,6 +1686,9 @@ sysctl_file(int *name, u_int namelen, ch
> 				/* not the pid we are looking for */
> 				continue;
> 			}
> +
> +			refcnt_take(&pr->ps_refcnt);
> +
> 			matched = 1;
> 			fdp = pr->ps_fd;
> 			if (pr->ps_textvp)
> @@ -1702,6 +1705,9 @@ sysctl_file(int *name, u_int namelen, ch
> 				FILLIT(fp, fdp, i, NULL, pr);
> 				FRELE(fp, p);
> 			}
> +
> +			refcnt_rele_wake(&pr->ps_refcnt);
> +
> 			/* pid is unique, stop searching */
> 			if (arg >= 0)
> 				break;
> @@ -1721,6 +1727,9 @@ sysctl_file(int *name, u_int namelen, ch
> 				/* not the uid we are looking for */
> 				continue;
> 			}
> +
> +			refcnt_take(&pr->ps_refcnt);
> +
> 			fdp = pr->ps_fd;
> 			if (fdp->fd_cdir)
> 				FILLIT(NULL, NULL, KERN_FILE_CDIR, fdp->fd_cdir, pr);
> @@ -1734,6 +1743,8 @@ sysctl_file(int *name, u_int namelen, ch
> 				FILLIT(fp, fdp, i, NULL, pr);
> 				FRELE(fp, p);
> 			}
> +
> +			refcnt_rele_wake(&pr->ps_refcnt);
> 		}
> 		break;
> 	default:
> Index: sys/sys/proc.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/proc.h,v
> diff -u -p -r1.367 proc.h
> --- sys/sys/proc.h	6 Aug 2024 08:44:54 -0000	1.367
> +++ sys/sys/proc.h	9 Aug 2024 11:51:30 -0000
> @@ -142,6 +142,8 @@ struct pinsyscall {
>  *	T	itimer_mtx
>  */
> struct process {
> +	struct refcnt ps_refcnt;
> +
> 	/*
> 	 * ps_mainproc is the original thread in the process.
> 	 * It's only still special for the handling of
>