kgr: ignore zombie tasks during the patchingkgraft

There is no special treatment of zombie tasks in kGraft. This means that
they can block the patching process almost infinitely if their
TIF_KGR_IN_PROGRESS flag is set, because they cannot run anymore and
cannot clear the flag.

Simple reproducer:

	# echo 'main() { if(fork()) sleep(300); }' > zombie.c
	# gcc -o zombie zombie.c
	# ./zombie &
	# ps xa | fgrep Z
	# zypper in -t pattern lp_sles
	... installation running
	# kgr status
	in_progress
	# kgr -vv blocking
	2705 [zombie]
	  [<ffffffff8105b877>] do_exit+0x6f7/0xa80
	  [<ffffffff8105bc79>] do_group_exit+0x39/0xa0
	  [<ffffffff8105bcf0>] __wake_up_parent+0x0/0x30
	  [<ffffffff8152dd09>] system_call_fastpath+0x16/0x1b
	  [<00007fd128f9c4f9>] 0x7fd128f9c4f9
	  [<ffffffffffffffff>] 0xffffffffffffffff

Let's ignore the zombie tasks in kgr_still_patching() check. Such task
is in TASK_DEAD state. We also need to be sure that the task is not
running anymore. Otherwise there would be unpleasant race conditions. So
also check task's ->on_cpu. It is set to 0 by the next task in
finish_lock_switch().

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/kgraft.c b/kernel/kgraft.c
index 08d27a47583b2d..fdf6b10ac23163 100644
--- a/kernel/kgraft.c
+++ b/kernel/kgraft.c
@@ -151,7 +151,13 @@ static bool kgr_still_patching(void)
 
 	read_lock(&tasklist_lock);
 	for_each_process_thread(p, t) {
-		if (kgr_task_in_progress(t)) {
+		/*
+		 * Ignore zombie tasks, that is task with ->state == TASK_DEAD.
+		 * We also need to check their ->on_cpu to be sure that they are
+		 * not running any code and they are really almost dead.
+		 */
+		if (kgr_task_in_progress(t) && (t->state != TASK_DEAD ||
+		    t->on_cpu != 0)) {
 			failed = true;
 			goto unlock;
 		}