From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752187AbdCNNWJ (ORCPT <rfc822;w@1wt.eu>);
        Tue, 14 Mar 2017 09:22:09 -0400
Received: from mx2.suse.de ([195.135.220.15]:40304 "EHLO mx2.suse.de"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751638AbdCNNQW (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 14 Mar 2017 09:16:22 -0400
X-Amavis-Alert: BAD HEADER SECTION, Duplicate header field: "References"
From: Jiri Slaby <jslaby@suse.cz>
To: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, Jiri Slaby <jslaby@suse.cz>
Subject: [PATCH 3.12 45/60] TTY: n_hdlc, fix lockdep false positive
Date: Tue, 14 Mar 2017 14:15:36 +0100
Message-Id: <a1b2f63f3024a235216883ab0c90b81bcedfc264.1489497268.git.jslaby@suse.cz>
X-Mailer: git-send-email 2.12.0
In-Reply-To: <d93cf67053e241539a1ef7c30ee8583022bc0e89.1489497268.git.jslaby@suse.cz>
References: <d93cf67053e241539a1ef7c30ee8583022bc0e89.1489497268.git.jslaby@suse.cz>
In-Reply-To: <cover.1489497268.git.jslaby@suse.cz>
References: <cover.1489497268.git.jslaby@suse.cz>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit e9b736d88af1a143530565929390cadf036dc799 upstream.

The class of 4 n_hdls buf locks is the same because a single function
n_hdlc_buf_list_init is used to init all the locks. But since
flush_tx_queue takes n_hdlc->tx_buf_list.spinlock and then calls
n_hdlc_buf_put which takes n_hdlc->tx_free_buf_list.spinlock, lockdep
emits a warning:
=============================================
[ INFO: possible recursive locking detected ]
4.3.0-25.g91e30a7-default #1 Not tainted
---------------------------------------------
a.out/1248 is trying to acquire lock:
 (&(&list->spinlock)->rlock){......}, at: [<ffffffffa01fd020>] n_hdlc_buf_put+0x20/0x60 [n_hdlc]

but task is already holding lock:
 (&(&list->spinlock)->rlock){......}, at: [<ffffffffa01fdc07>] n_hdlc_tty_ioctl+0x127/0x1d0 [n_hdlc]

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(&list->spinlock)->rlock);
  lock(&(&list->spinlock)->rlock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by a.out/1248:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff814c9eb0>] tty_ldisc_ref_wait+0x20/0x50
 #1:  (&(&list->spinlock)->rlock){......}, at: [<ffffffffa01fdc07>] n_hdlc_tty_ioctl+0x127/0x1d0 [n_hdlc]
...
Call Trace:
...
 [<ffffffff81738fd0>] _raw_spin_lock_irqsave+0x50/0x70
 [<ffffffffa01fd020>] n_hdlc_buf_put+0x20/0x60 [n_hdlc]
 [<ffffffffa01fdc24>] n_hdlc_tty_ioctl+0x144/0x1d0 [n_hdlc]
 [<ffffffff814c25c1>] tty_ioctl+0x3f1/0xe40
...

Fix it by initializing the spin_locks separately. This removes also
reduntand memset of a freshly kzallocated space.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 drivers/tty/n_hdlc.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 1b2db9a3038c..f26657c06870 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -159,7 +159,6 @@ struct n_hdlc {
 /*
  * HDLC buffer list manipulation functions
  */
-static void n_hdlc_buf_list_init(struct n_hdlc_buf_list *list);
 static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
 			   struct n_hdlc_buf *buf);
 static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *list);
@@ -855,10 +854,10 @@ static struct n_hdlc *n_hdlc_alloc(void)
 
 	memset(n_hdlc, 0, sizeof(*n_hdlc));
 
-	n_hdlc_buf_list_init(&n_hdlc->rx_free_buf_list);
-	n_hdlc_buf_list_init(&n_hdlc->tx_free_buf_list);
-	n_hdlc_buf_list_init(&n_hdlc->rx_buf_list);
-	n_hdlc_buf_list_init(&n_hdlc->tx_buf_list);
+	spin_lock_init(&n_hdlc->rx_free_buf_list.spinlock);
+	spin_lock_init(&n_hdlc->tx_free_buf_list.spinlock);
+	spin_lock_init(&n_hdlc->rx_buf_list.spinlock);
+	spin_lock_init(&n_hdlc->tx_buf_list.spinlock);
 	
 	/* allocate free rx buffer list */
 	for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
@@ -887,16 +886,6 @@ static struct n_hdlc *n_hdlc_alloc(void)
 }	/* end of n_hdlc_alloc() */
 
 /**
- * n_hdlc_buf_list_init - initialize specified HDLC buffer list
- * @list - pointer to buffer list
- */
-static void n_hdlc_buf_list_init(struct n_hdlc_buf_list *list)
-{
-	memset(list, 0, sizeof(*list));
-	spin_lock_init(&list->spinlock);
-}	/* end of n_hdlc_buf_list_init() */
-
-/**
  * n_hdlc_buf_put - add specified HDLC buffer to tail of specified list
  * @list - pointer to buffer list
  * @buf	- pointer to buffer
-- 
2.12.0