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

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

After commit dce8f8ed1de1 ("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/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions