[PATCH v3] kernel/exit: do panic earlier to get coredump if global init task exit

[qiwuchen55@gmail.com]

From: chenqiwu <chenqiwu@xiaomi.com>

When global init task get a chance to be killed, panic will happen in
later calling steps by do_exit()->exit_notify()->forget_original_parent()
->find_child_reaper() if all init threads have exited.

However, it's hard to extract the coredump of init task from a kernel
crashdump, since exit_mm() has released its mm before panic. In order
to get the backtrace of init task in userspace, it's better to do panic
earlier at the beginning of exitting route.

It's worth noting that we must take case of a multi-threaded init exitting
issue. We need the test for is_global_init() && group_dead to ensure that
it is all threads exiting and not just the current thread.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
changes in v3:
 - move panic into group_dead condition.
 - keep exitcode as the original code does.
 - fix logic error for comment.
---
 kernel/exit.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index bcbd598..7271e13 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -517,10 +517,6 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
 	}
 
 	write_unlock_irq(&tasklist_lock);
-	if (unlikely(pid_ns == &init_pid_ns)) {
-		panic("Attempted to kill init! exitcode=0x%08x\n",
-			father->signal->group_exit_code ?: father->exit_code);
-	}
 
 	list_for_each_entry_safe(p, n, dead, ptrace_entry) {
 		list_del_init(&p->ptrace_entry);
@@ -766,6 +762,15 @@ void __noreturn do_exit(long code)
 	acct_update_integrals(tsk);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
+		/*
+		 * If the last thread of global init exit, do panic
+		 * immeddiately to get the coredump to find any clue
+		 * for init task in userspace.
+		 */
+		if (unlikely(is_global_init(tsk)))
+			panic("Attempted to kill init! exitcode=0x%08x\n",
+				tsk->signal->group_exit_code ?: (int)code);
+
 #ifdef CONFIG_POSIX_TIMERS
 		hrtimer_cancel(&tsk->signal->real_timer);
 		exit_itimers(tsk->signal);
-- 
1.9.1