[PATCH] Call exit_itimers from do_exit, not __exit_signal

When I originally moved exit_itimers into __exit_signal, that was the only
place where we could reliably know it was the last thread in the group
dying, without races.  Since then we've gotten the signal_struct.live
counter, and do_exit can reliably do group-wide cleanup work.

This patch moves the call to do_exit, where it's made without locks.  This
avoids the deadlock issues that the old __exit_signal code's comment talks
about, and the one that Oleg found recently with process CPU timers.

[ This replaces e03d13e985d48ac4885382c9e3b1510c78bd047f, which is why
  it was just reverted. ]

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/exit.c

@@ -843,6 +843,7 @@ fastcall NORET_TYPE void do_exit(long code)
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
  		del_timer_sync(&tsk->signal->real_timer);
+		exit_itimers(tsk->signal);
 		acct_process(code);
 	}
 	exit_mm(tsk);

kernel/posix-timers.c

@@ -1157,7 +1157,7 @@ retry_delete:
 }
 
 /*
- * This is called by __exit_signal, only when there are no more
+ * This is called by do_exit or de_thread, only when there are no more
  * references to the shared signal_struct.
  */
 void exit_itimers(struct signal_struct *sig)

kernel/signal.c

@@ -397,20 +397,8 @@ void __exit_signal(struct task_struct *tsk)
 	flush_sigqueue(&tsk->pending);
 	if (sig) {
 		/*
-		 * We are cleaning up the signal_struct here.  We delayed
-		 * calling exit_itimers until after flush_sigqueue, just in
-		 * case our thread-local pending queue contained a queued
-		 * timer signal that would have been cleared in
-		 * exit_itimers.  When that called sigqueue_free, it would
-		 * attempt to re-take the tasklist_lock and deadlock.  This
-		 * can never happen if we ensure that all queues the
-		 * timer's signal might be queued on have been flushed
-		 * first.  The shared_pending queue, and our own pending
-		 * queue are the only queues the timer could be on, since
-		 * there are no other threads left in the group and timer
-		 * signals are constrained to threads inside the group.
+		 * We are cleaning up the signal_struct here.
 		 */
-		exit_itimers(sig);
 		exit_thread_group_keys(sig);
 		kmem_cache_free(signal_cachep, sig);
 	}