proc: add tree ref count

Owning the reference prevents reuse of the struct proc.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D57492

sys/kern/kern_exit.c

@@ -1107,7 +1107,7 @@ proc_reap(struct thread *td, struct proc *p, int *status, int options)
 
 	KASSERT(FIRST_THREAD_IN_PROC(p),
 	    ("proc_reap: no residual thread!"));
-	uma_zfree(proc_zone, p);
+	PROC_TREE_UNREF(p);
 	atomic_add_int(&nprocs, -1);
 }
 

sys/kern/kern_fork.c

@@ -1051,6 +1051,7 @@ fork1(struct thread *td, struct fork_req *fr)
 		pages = kstack_pages;
 	/* Allocate new proc. */
 	newproc = uma_zalloc(proc_zone, M_WAITOK);
+	PROC_TREE_REF(newproc);
 	td2 = FIRST_THREAD_IN_PROC(newproc);
 	if (td2 == NULL) {
 		td2 = thread_alloc(pages);
@@ -1131,7 +1132,7 @@ fork1(struct thread *td, struct fork_req *fr)
 fail2:
 	if (vm2 != NULL)
 		vmspace_free(vm2);
-	uma_zfree(proc_zone, newproc);
+	PROC_TREE_UNREF(newproc);
 	if ((flags & RFPROCDESC) != 0 && fp_procdesc != NULL) {
 		fdclose(td, fp_procdesc, *fr->fr_pd_fd);
 		fdrop(fp_procdesc, td);

sys/kern/kern_proc.c

@@ -279,6 +279,7 @@ proc_init(void *mem, int size, int flags)
 	p->p_pgrp = NULL;
 	TAILQ_INIT(&p->p_kqtim_stop);
 	STAILQ_INIT(&p->p_ktr);
+	refcount_init(&p->p_tree_refcnt, 0);
 	return (0);
 }
 

sys/sys/proc.h

@@ -779,6 +779,7 @@ struct proc {
 	TAILQ_HEAD(, kq_timer_cb_data)	p_kqtim_stop;	/* (c) */
 	LIST_ENTRY(proc) p_jaillist;	/* (d) Jail process linkage. */
 	u_int		p_asig;		/* (c) ASYNCEXIT pending signal. */
+	u_int		p_tree_refcnt;	/* (e) proctree refcount */
 };
 
 #define	p_session	p_pgrp->pg_session
@@ -804,6 +805,12 @@ struct proc {
 #define	PROC_PROFUNLOCK(p)	mtx_unlock_spin(&(p)->p_profmtx)
 #define	PROC_PROFLOCK_ASSERT(p, type)	mtx_assert(&(p)->p_profmtx, (type))
 
+#define	PROC_TREE_REF(p)	refcount_acquire(&(p)->p_tree_refcnt)
+#define	PROC_TREE_UNREF(p)	do {					\
+	if (refcount_release(&(p)->p_tree_refcnt))			\
+		uma_zfree(proc_zone, p);				\
+} while (0)
+
 /* These flags are kept in p_flag. */
 #define	P_ADVLOCK	0x00000001	/* Process may hold a POSIX advisory
 					   lock. */