[PATCH] fix kill_proc_info() vs fork() theoretical race

copy_process:

	attach_pid(p, PIDTYPE_PID, p->pid);
	attach_pid(p, PIDTYPE_TGID, p->tgid);

What if kill_proc_info(p->pid) happens in between?

copy_process() holds current->sighand.siglock, so we are safe
in CLONE_THREAD case, because current->sighand == p->sighand.

Otherwise, p->sighand is unlocked, the new process is already
visible to the find_task_by_pid(), but have a copy of parent's
'struct pid' in ->pids[PIDTYPE_TGID].

This means that __group_complete_signal() may hang while doing

	do ... while (next_thread() != p)

We can solve this problem if we reverse these 2 attach_pid()s:

	attach_pid() does wmb()

	group_send_sig_info() calls spin_lock(), which
	provides a read barrier. // Yes ?

I don't think we can hit this race in practice, but still.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/fork.c

@@ -1173,8 +1173,6 @@ static task_t *copy_process(unsigned long clone_flags,
 	if (unlikely(p->ptrace & PT_PTRACED))
 		__ptrace_link(p, current->parent);
 
-	attach_pid(p, PIDTYPE_PID, p->pid);
-	attach_pid(p, PIDTYPE_TGID, p->tgid);
 	if (thread_group_leader(p)) {
 		p->signal->tty = current->signal->tty;
 		p->signal->pgrp = process_group(current);
@@ -1184,6 +1182,8 @@ static task_t *copy_process(unsigned long clone_flags,
 		if (p->pid)
 			__get_cpu_var(process_counts)++;
 	}
+	attach_pid(p, PIDTYPE_TGID, p->tgid);
+	attach_pid(p, PIDTYPE_PID, p->pid);
 
 	nr_threads++;
 	total_forks++;