[PATCH] tasklist left locked

On 4-way SMP, about one reboot in twenty hangs while killing processes:
exit needs exclusive tasklist_lock, but something still holds read_lock.
do_signal_stop race case misses unlock, and fixing it fixes the symptom.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/signal.c

@@ -1745,6 +1745,7 @@ do_signal_stop(int signr)
 			 * with another processor delivering a stop signal,
 			 * then the SIGCONT that wakes us up should clear it.
 			 */
+			read_unlock(&tasklist_lock);
 			return 0;
 		}