introduce __next_thread(), fix next_tid() vs exec() race

Patch series "introduce __next_thread(), change next_thread()".

After commit dce8f8ed1d ("document while_each_thread(), change
first_tid() to use for_each_thread()") + this series

1. We have only one lockless user of next_thread(), task_group_seq_get_next().
   I think it should be changed too.

2. We have only one user of task_struct->thread_group, thread_group_empty().
   The next patches will change thread_group_empty() and kill ->thread_group.


This patch (of 2):

next_tid(start) does:

	rcu_read_lock();
	if (pid_alive(start)) {
		pos = next_thread(start);
		if (thread_group_leader(pos))
			pos = NULL;
		else
			get_task_struct(pos);

it should return pos = NULL when next_thread() wraps to the 1st thread
in the thread group, group leader, and the thread_group_leader() check
tries to detect this case.

But this can race with exec. To simplify, suppose we have a main thread
M and a single sub-thread T, next_tid(T) should return NULL.

Now suppose that T execs. If next_tid(T) is called after T changes the
leadership and before it does release_task() which removes the old leader
from list, then next_thread() returns M and thread_group_leader(M) = F.

Lockless use of next_thread() should be avoided. After this change only
task_group_seq_get_next() does this, and I believe it should be changed
as well.

Link: https://lkml.kernel.org/r/20230824143112.GA31208@redhat.com
Link: https://lkml.kernel.org/r/20230824143142.GA31222@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/proc/base.c

@@ -3840,10 +3840,8 @@ static struct task_struct *next_tid(struct task_struct *start)
 	struct task_struct *pos = NULL;
 	rcu_read_lock();
 	if (pid_alive(start)) {
-		pos = next_thread(start);
-		if (thread_group_leader(pos))
-			pos = NULL;
-		else
+		pos = __next_thread(start);
+		if (pos)
 			get_task_struct(pos);
 	}
 	rcu_read_unlock();

include/linux/sched/signal.h

@@ -715,6 +715,17 @@ bool same_thread_group(struct task_struct *p1, struct task_struct *p2)
 	return p1->signal == p2->signal;
 }
 
+/*
+ * returns NULL if p is the last thread in the thread group
+ */
+static inline struct task_struct *__next_thread(struct task_struct *p)
+{
+	return list_next_or_null_rcu(&p->signal->thread_head,
+					&p->thread_node,
+					struct task_struct,
+					thread_node);
+}
+
 static inline struct task_struct *next_thread(const struct task_struct *p)
 {
 	return list_entry_rcu(p->thread_group.next,