Re: [PATCH] fix race copy_process() vs de_thread()

[Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>]

Roland McGrath wrote:
> I'm not sure I follow the problem scenario you are citing.
> 
> The thread in copy_process() will return -ERESTARTNOINTR
> after calling cleanup_signal(), which does properly decrement
> sig->count to return it to the state before the copy_process() call.
> Then this thread will get to signal handling, dequeue its SIGKILL from
> zap_other_threads(), and die itself.  When it's finally reaped, by itself
> in exit_notify(), or by de_thread() in the case of a replaced group_leader,
> the normal __exit_signal() will do that group_exit_task logic.
> 
> What part of this sequence fails to occur in your tests?

The point is that de_thread() waits until other thread calls wake_up_process().
In __exit_signal() when sig->count == 2, the thread calls wake_up_process(),
and then de_thread() will continue. However if another thread is during
copy_process(), the sig->count is incremented at copy_signal(). That makes
no wake_up_process().

The scenario flow is like this;

thread A          thread B    thread C      sig->count
   |                 |           |              3
  clone()          exec()        |              |
  copy_process()   de_thread()   |              |
  copy_signal()      |           |              4
   |                zap          |              4
  KILL               |          KILL            4
   |                 |          __exit_signal() 3 no wake up
  clean_signal()     |                          2 no wake up
   |                 |                             during copy_process()
  exit               |
                never wake up

thanks,
Hiroshi