exec: kill unsafe BUG_ON(sig->count) checks

de_thread:

	if (atomic_read(&oldsighand->count) <= 1)
		BUG_ON(atomic_read(&sig->count) != 1);

This is not safe without the rmb() in between.  The results of two
correctly ordered __exit_signal()->atomic_dec_and_test()'s could be seen
out of order on our CPU.

The same is true for the "thread_group_empty()" case, __unhash_process()'s
changes could be seen before atomic_dec_and_test(&sig->count).

On some platforms (including i386) atomic_read() doesn't provide even the
compiler barrier, in that case these checks are simply racy.

Remove these BUG_ON()'s. Alternatively, we can do something like

	BUG_ON( ({ smp_rmb(); atomic_read(&sig->count) != 1; }) );
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -784,7 +784,6 @@ static int de_thread(struct task_struct *tsk)
 	 * and we can just re-use it all.
 	 */
 	if (atomic_read(&oldsighand->count) <= 1) {
-		BUG_ON(atomic_read(&sig->count) != 1);
 		signalfd_detach(tsk);
 		exit_itimers(sig);
 		return 0;
@@ -929,8 +928,6 @@ static int de_thread(struct task_struct *tsk)
 	if (leader)
 		release_task(leader);
 
-	BUG_ON(atomic_read(&sig->count) != 1);
-
 	if (atomic_read(&oldsighand->count) == 1) {
 		/*
 		 * Now that we nuked the rest of the thread group,