[PATCH] TTY: Fix the missing lock for the TTY ldisc buffer

[Denis Du <dudenis2000@yahoo.ca>]

Hi, Guys:
It was found that the 3.12 kernel tty layer will lose or corrupt data 
when have a full-duplex communication, especially in high baud rate, for 
example 230k for my OMAP5 uart. Eventually I found there is lock missing 
between copy data to ldisc layer buffer and copy data from the same 
buffer to user space. I believe this issue existed since 3.8 
kernel(since this kernel , it start to remove most of the spin-locks) 
and I didn't find any fix even through 3.17 kernel. This patch was 
tested to be works great with no any data loss again on 3.12 kernel.

This patch was built for the latest kernel, but I cannot test it. 
Somebody may give a test.

I did try to use the existed lock atomic_read_lock, but it doesn’t work.



Signed-off-by: Hui Du <dudenis2000@yahoo.ca>

---
  drivers/tty/n_tty.c |    9 ++++++++-
  1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 2e900a9..6e5c6ae 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -125,6 +125,7 @@ struct n_tty_data {
  
  	struct mutex atomic_read_lock;
  	struct mutex output_lock;
+	struct mutex read_buf_lock;
  };
  
  static inline size_t read_cnt(struct n_tty_data *ldata)
@@ -1691,7 +1692,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
  	int room, n, rcvd = 0;
  
  	down_read(&tty->termios_rwsem);
-
+	mutex_lock(&ldata->read_buf_lock);
  	while (1) {
  		room = receive_room(tty);
  		n = min(count, room);
@@ -1710,6 +1711,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
  
  	tty->receive_room = room;
  	n_tty_check_throttle(tty);
+	mutex_unlock(&ldata->read_buf_lock);
  	up_read(&tty->termios_rwsem);
  
  	return rcvd;
@@ -1876,6 +1878,7 @@ static int n_tty_open(struct tty_struct *tty)
  	ldata->overrun_time = jiffies;
  	mutex_init(&ldata->atomic_read_lock);
  	mutex_init(&ldata->output_lock);
+	mutex_init(&ldata->read_buf_lock);
  
  	tty->disc_data = ldata;
  	reset_buffer_flags(tty->disc_data);
@@ -1939,6 +1942,7 @@ static int copy_from_read_buf(struct tty_struct *tty,
  	size_t tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
  
  	retval = 0;
+	mutex_lock(&ldata->read_buf_lock);
  	n = min(read_cnt(ldata), N_TTY_BUF_SIZE - tail);
  	n = min(*nr, n);
  	if (n) {
@@ -1954,6 +1958,7 @@ static int copy_from_read_buf(struct tty_struct *tty,
  		*b += n;
  		*nr -= n;
  	}
+	mutex_unlock(&ldata->read_buf_lock);
  	return retval;
  }
  
@@ -1992,6 +1997,7 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
  	bool eof_push = 0;
  
  	/* N.B. avoid overrun if nr == 0 */
+	mutex_lock(&ldata->read_buf_lock);
  	n = min(*nr, read_cnt(ldata));
  	if (!n)
  		return 0;
@@ -2052,6 +2058,7 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
  			ldata->push = 0;
  		tty_audit_push(tty);
  	}
+	mutex_unlock(&ldata->read_buf_lock);
  	return eof_push ? -EAGAIN : 0;
  }
  
-- 
1.7.10.4