[PATCH] fill_tgid: fix task_struct leak and possible oops

1. fill_tgid() forgets to do put_task_struct(first).

2. release_task(first) can happen after fill_tgid() drops tasklist_lock,
   it is unsafe to dereference first->signal.

This is a temporary fix, imho the locking should be reworked.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Jay Lan <jlan@sgi.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/taskstats.c

@@ -237,14 +237,17 @@ static int fill_tgid(pid_t tgid, struct task_struct *tgidtsk,
 	} else
 		get_task_struct(first);
 
-	/* Start with stats from dead tasks */
-	spin_lock_irqsave(&first->signal->stats_lock, flags);
-	if (first->signal->stats)
-		memcpy(stats, first->signal->stats, sizeof(*stats));
-	spin_unlock_irqrestore(&first->signal->stats_lock, flags);
 
 	tsk = first;
 	read_lock(&tasklist_lock);
+	/* Start with stats from dead tasks */
+	if (first->signal) {
+		spin_lock_irqsave(&first->signal->stats_lock, flags);
+		if (first->signal->stats)
+			memcpy(stats, first->signal->stats, sizeof(*stats));
+		spin_unlock_irqrestore(&first->signal->stats_lock, flags);
+	}
+
 	do {
 		if (tsk->exit_state == EXIT_ZOMBIE && thread_group_leader(tsk))
 			continue;
@@ -264,7 +267,7 @@ static int fill_tgid(pid_t tgid, struct task_struct *tgidtsk,
 	 * Accounting subsytems can also add calls here to modify
 	 * fields of taskstats.
 	 */
-
+	put_task_struct(first);
 	return 0;
 }