[Devel] [PATCH RHEL7 COMMIT] ms/TTY: n_hdlc, fix lockdep false positive

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.10.2.vz7.29.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.10.2.vz7.29.3
------>
commit 7dcc64d7bb8a3dff22453d99046ade62cf72b28c
Author: Jiri Slaby <jslaby at suse.cz>
Date:   Thu Nov 26 19:28:26 2015 +0100

    ms/TTY: n_hdlc, fix lockdep false positive
    
    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 at suse.cz>
    Reported-by: Dmitry Vyukov <dvyukov at google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
    (cherry picked from commit e9b736d88af1a143530565929390cadf036dc799)
    
    Cleanup, easier to apply next patch with it.
    https://jira.sw.ru/browse/PSBM-61963
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 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 1b2db9a..f26657c 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