signals: make task_struct->signal immutable/refcountable

We have a lot of problems with accessing task_struct->signal, it can
"disappear" at any moment.  Even current can't use its ->signal safely
after exit_notify().  ->siglock helps, but it is not convenient, not
always possible, and sometimes it makes sense to use task->signal even
after this task has already dead.

This patch adds the reference counter, sigcnt, into signal_struct.  This
reference is owned by task_struct and it is dropped in
__put_task_struct().  Perhaps it makes sense to export
get/put_signal_struct() later, but currently I don't see the immediate
reason.

Rename __cleanup_signal() to free_signal_struct() and unexport it.  With
the previous changes it does nothing except kmem_cache_free().

Change __exit_signal() to not clear/free ->signal, it will be freed when
the last reference to any thread in the thread group goes away.

Note:
	- when the last thead exits signal->tty can point to nowhere, see
	  the next patch.

	- with or without this patch signal_struct->count should go away,
	  or at least it should be "int nr_threads" for fs/proc. This will
	  be addressed later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a95a2455cebe..32e309df408c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -527,6 +527,7 @@ struct thread_group_cputimer {
  * the locking of signal_struct.
  */
 struct signal_struct {
+        atomic_t                sigcnt;
         atomic_t                count;
         atomic_t                live;
 
@@ -2101,7 +2102,6 @@ extern void flush_thread(void);
 extern void exit_thread(void);
 
 extern void exit_files(struct task_struct *);
-extern void __cleanup_signal(struct signal_struct *);
 extern void __cleanup_sighand(struct sighand_struct *);
 
 extern void exit_itimers(struct signal_struct *);