ate(&tid, NULL, null_thread, NULL);
			}

			do {
				ret = waitpid(pid, &status, 0);
			} while (ret == -1 && errno == EINTR);
		}

		return 0;
	}

quickly creates an unkillable task.

If copy_process(CLONE_THREAD) races with de_thread()
copy_signal()->atomic(signal->count) breaks the signal->notify_count
logic, and the execing thread can hang forever in kernel space.

Change copy_process() to increment count/live only when we know for sure
we can't fail.  In this case the forked thread will take care of its
reference to signal correctly.

If copy_process() fails, check CLONE_THREAD flag.  If it it set - do
nothing, the counters were not changed and current belongs to the same
thread group.  If it is not set, ->signal must be released in any case
(and ->count must be == 1), the forked child is the only thread in the
thread group.

We need more cleanups here, in particular signal->count should not be used
by de_thread/__exit_signal at all.  This patch only fixes the bug.

Reported-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Tested-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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