[PATCH] prctl: propagate has_child_subreaper flag to every descendant

[Pavel Tikhomirov]

If process forks some children when it has is_child_subreaper
flag enabled they will inherit has_child_subreaper flag - first
group, when is_child_subreaper is disabled forked children will
not inherit it - second group. So child-subreaper does not reparent
all his descendants when their parents die. Having these two
differently behaving groups can lead to confusion. Also it is
a problem for CRIU, as when we restore process tree we need to
somehow determine which descendants belong to which group and
much harder - to put them exactly to these group.

To simplify these we can add a propagation of has_child_subreaper
flag on PR_SET_CHILD_SUBREAPER, walking all descendants of child-
subreaper to setup has_child_subreaper flag.

In common cases when process like systemd first sets itself to
be a child-subreaper and only after that forks its services, we will
have zero-length list of descendants to walk. Testing with binary
subtree of 2^15 processes prctl took < 0.007 sec and has shown close
to linear dependency(~0.2 * n * usec) on lower numbers of processes.

Using csr_descendant list to collect descendants and do tree walk
without recursion.

Optimize:

a) When descendant already has has_child_subreaper flag all his subtree
has it too already.

b) When some descendant is child_reaper, it's subtree is in different
pidns from us(original child-subreaper) and processes from other pidns
will never reparent to us.

So we can skip their(a,b) subtree from walk.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
---
 include/linux/sched.h |  2 ++
 kernel/sys.c          | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4d19052..9cb44c4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1715,6 +1715,8 @@ struct task_struct {
 	struct signal_struct *signal;
 	struct sighand_struct *sighand;
 
+	struct list_head csr_descendant;
+
 	sigset_t blocked, real_blocked;
 	sigset_t saved_sigmask;	/* restored if set_restore_sigmask() was used */
 	struct sigpending pending;
diff --git a/kernel/sys.c b/kernel/sys.c
index 842914e..05b6d7d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2063,6 +2063,55 @@ static int prctl_get_tid_address(struct task_struct *me, int __user **tid_addr)
 }
 #endif
 
+static DEFINE_SPINLOCK(descendants_lock);
+
+static void prctl_set_child_subreaper(struct task_struct *reaper, bool arg2)
+{
+	LIST_HEAD(descendants);
+
+	reaper->signal->is_child_subreaper = arg2;
+	if (!arg2)
+		return;
+
+	spin_lock(&descendants_lock);
+	read_lock(&tasklist_lock);
+
+	list_add(&reaper->csr_descendant, &descendants);
+
+	while (!list_empty(&descendants)) {
+		struct task_struct *tsk;
+		struct task_struct *p;
+
+		tsk = list_first_entry(&descendants, struct task_struct,
+				csr_descendant);
+
+		list_for_each_entry(p, &tsk->children, sibling) {
+			/*
+			 * If tsk has has_child_subreaper - all its decendants
+			 * already have these flag too and new decendants will
+			 * inherit it on fork, so nothing to be done here.
+			 */
+			if (p->signal->has_child_subreaper)
+				continue;
+
+			/*
+			 * If we've found child_reaper - skip descendants in
+			 * it's subtree as they will never get out pidns
+			 */
+			if (is_child_reaper(task_pid(p)))
+				continue;
+
+			p->signal->has_child_subreaper = 1;
+			list_add(&p->csr_descendant, &descendants);
+		}
+
+		list_del_init(&tsk->csr_descendant);
+	}
+
+	read_unlock(&tasklist_lock);
+	spin_unlock(&descendants_lock);
+}
+
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
@@ -2213,7 +2262,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		error = prctl_get_tid_address(me, (int __user **)arg2);
 		break;
 	case PR_SET_CHILD_SUBREAPER:
-		me->signal->is_child_subreaper = !!arg2;
+		prctl_set_child_subreaper(me, !!arg2);
 		break;
 	case PR_GET_CHILD_SUBREAPER:
 		error = put_user(me->signal->is_child_subreaper,
-- 
2.9.3