[CRIU] [PATCHv4 2/3] restore: Fix deadlock when helper's child dies

[Dmitry Safonov]

Since commit ced9c529f687 ("restore: fix race with helpers' kids dying
too early"), we block SIGCHLD in helper tasks before CR_STATE_RESTORE.
This way we avoided default criu sighandler as it doesn't expect that
childs may die.

This is very racy as we wait on futex for another stage to be started,
but the next stage may start only when all the tasks complete previous
stage. If some children of helper dies, the helper may already have
blocked SIGCHLD and have started sleeping on the futex. Then the next
stage never comes and no one reads a pending SIGCHLD for helper.

A customer met this situation on the node, where the following
(non-related) problem has occured:
Unable to send a fin packet: libnet_write_raw_ipv6(): -1 bytes written (Network is unreachable)
Then child criu of the helper has exited with error-code and the
lockup has happened.

While we could fix it by aborting futex in the end of
restore_task_with_children() for each (non-root also) tasks,
that would be not completely correct:
1. All futex-waiting tasks will wake up after that and they
   may not expect that some tasks are on the previous stage,
   so they will spam into logs with unrelated errors and may
   also die painfully.
2. Child may die and miss aborting of the futex due to:
   o segfault
   o OOM killer
   o User-sended SIGKILL
   o Other error-path we forgot to cover with abort futex

To fix this deadlock in TASK_HELPER, as suggested-by Kirill,
let's check if there are children deaths expected - if there
isn't any, don't block SIGCHLD, otherwise wait() and check if
death was on expected stage of restore (not CR_STATE_RESTORE).

Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov at virtuozzo.com>
---
 criu/cr-restore.c | 99 ++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 77 insertions(+), 22 deletions(-)

diff --git a/criu/cr-restore.c b/criu/cr-restore.c
index 9143cd310fed..59099cc84372 100644
--- a/criu/cr-restore.c
+++ b/criu/cr-restore.c
@@ -1207,6 +1207,82 @@ out:
 	return ret;
 }
 
+/*
+ * Find if there are children which are zombies or helpers - processes
+ * which are expected to die during the restore.
+ */
+static bool child_death_expected(void)
+{
+	struct pstree_item *pi;
+
+	list_for_each_entry(pi, &current->children, sibling) {
+		switch (pi->pid->state) {
+		case TASK_DEAD:
+		case TASK_HELPER:
+			return true;
+		}
+	}
+
+	return false;
+}
+
+/*
+ * Restore a helper process - artificially created by criu
+ * to restore attributes of process tree.
+ * - sessions for each leaders are dead
+ * - process groups with dead leaders
+ * - dead tasks for which /proc/<pid>/... is opened by restoring task
+ * - whatnot
+ */
+static int restore_one_helper(void)
+{
+	siginfo_t info;
+
+	if (prepare_fds(current))
+		return -1;
+
+	if (!child_death_expected()) {
+		/*
+		 * Restoree has no children that should die, during restore,
+		 * wait for the next stage on futex.
+		 * The default SIGCHLD handler will handle an unexpected
+		 * child's death and abort the restore if someone dies.
+		 */
+		restore_finish_stage(task_entries, CR_STATE_RESTORE);
+		return 0;
+	}
+
+	/*
+	 * The restoree has children which will die - decrement itself from
+	 * nr. of tasks processing the stage and wait for anyone to die.
+	 * Tasks may die only when they're on the following stage.
+	 * If one dies earlier - that's unexpected - treat it as an error
+	 * and abort the restore.
+	 */
+	if (block_sigmask(NULL, SIGCHLD))
+		return -1;
+
+	/* Finish CR_STATE_RESTORE, but do not wait for the next stage. */
+	futex_dec_and_wake(&task_entries->nr_in_progress);
+
+	if (waitid(P_ALL, 0, &info, WEXITED | WNOWAIT)) {
+		pr_perror("Failed to wait\n");
+		return -1;
+	}
+
+	if (futex_get(&task_entries->start) == CR_STATE_RESTORE) {
+		pr_err("Child %d died too early\n", info.si_pid);
+		return -1;
+	}
+
+	if (wait_on_helpers_zombies()) {
+		pr_err("Failed to wait on helpers and zombies\n");
+		return -1;
+	}
+
+	return 0;
+}
+
 static int restore_one_task(int pid, CoreEntry *core)
 {
 	int i, ret;
@@ -1218,32 +1294,11 @@ static int restore_one_task(int pid, CoreEntry *core)
 	else if (current->pid->state == TASK_DEAD)
 		ret = restore_one_zombie(core);
 	else if (current->pid->state == TASK_HELPER) {
-		sigset_t blockmask, oldmask;
-
-		if (prepare_fds(current))
-			return -1;
-
-		sigemptyset(&blockmask);
-		sigaddset(&blockmask, SIGCHLD);
-
-		if (sigprocmask(SIG_BLOCK, &blockmask, &oldmask) == -1) {
-			pr_perror("Can not set mask of blocked signals");
-			return -1;
-		}
-
-		restore_finish_stage(task_entries, CR_STATE_RESTORE);
-
+		ret = restore_one_helper();
 		close_image_dir();
 		close_proc();
 		for (i = SERVICE_FD_MIN + 1; i < SERVICE_FD_MAX; i++)
 			close_service_fd(i);
-
-		if (wait_on_helpers_zombies()) {
-			pr_err("failed to wait on helpers and zombies\n");
-			ret = -1;
-		} else {
-			ret = 0;
-		}
 	} else {
 		pr_err("Unknown state in code %d\n", (int)core->tc->task_state);
 		ret = -1;
-- 
2.13.3