[PATCH] procfs: Fix listing of /proc/NOT_A_TGID/task

Listing /proc/PID/task were PID is not a TGID should not result in
duplicated entries.

	[g ~]$ pidof thunderbird-bin
	2751
	[g ~]$ ls /proc/2751/task
	2751  2770  2771  2824  2826  2834  2835  2851  2853
	[g ~]$ ls /proc/2770/task
	2751  2770  2771  2824  2826  2834  2835  2851  2853
	2770  2771  2824  2826  2834  2835  2851  2853
	[g ~]$
Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2328,13 +2328,23 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
 {
 	struct dentry *dentry = filp->f_path.dentry;
 	struct inode *inode = dentry->d_inode;
-	struct task_struct *leader = get_proc_task(inode);
+	struct task_struct *leader = NULL;
 	struct task_struct *task;
 	int retval = -ENOENT;
 	ino_t ino;
 	int tid;
 	unsigned long pos = filp->f_pos;  /* avoiding "long long" filp->f_pos */
 
+	task = get_proc_task(inode);
+	if (!task)
+		goto out_no_task;
+	rcu_read_lock();
+	if (pid_alive(task)) {
+		leader = task->group_leader;
+		get_task_struct(leader);
+	}
+	rcu_read_unlock();
+	put_task_struct(task);
 	if (!leader)
 		goto out_no_task;
 	retval = 0;