[CRIU] [PATCH] restore: define root_as_sibling before using it

[Tycho Andersen]

root_as_sibling is used in criu_signals_setup(), but was only defined later
(when forking the root task for the first time). This meant that the
SA_NOCLDSTOP was never masked off, which meant SIGCHLD was never delivered
after ptracing the root task. Thus, when the a child of the root task died
(e.g. from cr_system), the root task sat in PTRACE_STOP, and the restore task
never PTRACE_CONT'd, resulting in a deadlock.

We also drop the pdeath_sig constraint from setting root_as_sibling when in
--restore-detached mode; in --restore-detached we /always/ need to have
root_as_sibling, but we only need to clone the parent if pdeath_sig is set and
we want to restore the task as alive.

v2: re-work the condition for CLONE_PARENT

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
---
 cr-restore.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/cr-restore.c b/cr-restore.c
index 2735d0d..fe6c798 100644
--- a/cr-restore.c
+++ b/cr-restore.c
@@ -956,25 +956,21 @@ struct cr_clone_arg {
 static void maybe_clone_parent(struct pstree_item *item,
 			      struct cr_clone_arg *ca)
 {
+	/*
+	 * zdtm runs in kernel 3.11, which has the problem described below. We
+	 * avoid this by including the pdeath_sig test. Once users/zdtm migrate
+	 * off of 3.11, this condition can be simplified to just test
+	 * root_as_sibling.
+	 */
 	if (opts.swrk_restore ||
 	    (opts.restore_detach && ca->core->thread_core->pdeath_sig)) {
 		/*
-		 * This means we're called from lib's criu_restore_child().
-		 * In that case create the root task as the child one to+
-		 * the caller. This is the only way to correctly restore the
-		 * pdeath_sig of the root task. But also looks nice.
-		 *
-		 * Alternatively, if we are --restore-detached, a similar trick is
-		 * needed to correctly restore pdeath_sig and prevent processes from
-		 * dying once restored.
-		 *
 		 * There were a problem in kernel 3.11 -- CLONE_PARENT can't be
 		 * set together with CLONE_NEWPID, which has been solved in further
 		 * versions of the kernels, but we treat 3.11 as a base, so at
 		 * least warn a user about potential problems.
 		 */
 		item->rst->clone_flags |= CLONE_PARENT;
-		root_as_sibling = 1;
 		if (item->rst->clone_flags & CLONE_NEWPID)
 			pr_warn("Set CLONE_PARENT | CLONE_NEWPID but it might cause restore problem,"
 				"because not all kernels support such clone flags combinations!\n");
@@ -1792,6 +1788,20 @@ int cr_restore_tasks(void)
 {
 	int ret = -1;
 
+	if (opts.swrk_restore || opts.restore_detach) {
+		/*
+		 * This means we're called from lib's criu_restore_child().
+		 * In that case create the root task as the child one to+
+		 * the caller. This is the only way to correctly restore the
+		 * pdeath_sig of the root task. But also looks nice.
+		 *
+		 * Alternatively, if we are --restore-detached, a similar trick is
+		 * needed to correctly restore pdeath_sig and prevent processes from
+		 * dying once restored.
+		 */
+		root_as_sibling = 1;
+	}
+
 	if (cr_plugin_init())
 		return -1;
 
-- 
1.9.1