[PATCH] TTY: missing a lock to access the ldisk buffer

[Denis Du <dudenis2000@yahoo.ca>]

Hi, Guys:
I found that the 3.12 kernel tty layer will lose or corrupt data when having a full-duplex communication, especcially in high baudrate, for example 230k for my OMAP5 uart. Eventually I found there is a lock missing between copy data to ldisc layer buffer and copy data from the same buffer to user space. I believe this issue have been existing 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.


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    2014-10-16 16:39:35.909350338 -0400
+++ drivers/tty/n_tty.c    2014-10-16 16:49:00.004930469 -0400
@@ -124,6 +124,7 @@

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)
@@ -1686,9 +1687,11 @@
char *fp, int count)
{
int room, n;
+    struct n_tty_data *ldata = tty->disc_data;

down_read(&tty->termios_rwsem);

+    mutex_lock(&ldata->read_buf_lock);
while (1) {
room = receive_room(tty);
n = min(count, room);
@@ -1703,6 +1706,7 @@

tty->receive_room = room;
n_tty_check_throttle(tty);
+    mutex_unlock(&ldata->read_buf_lock);
up_read(&tty->termios_rwsem);
}

@@ -1713,7 +1717,7 @@
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);
@@ -1732,6 +1736,7 @@

tty->receive_room = room;
n_tty_check_throttle(tty);
+    mutex_unlock(&ldata->read_buf_lock);
up_read(&tty->termios_rwsem);

return rcvd;
@@ -1880,6 +1885,7 @@
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);
@@ -1945,6 +1951,8 @@
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) {
@@ -1960,6 +1968,7 @@
*b += n;
*nr -= n;
}
+    mutex_unlock(&ldata->read_buf_lock);
return retval;
}

@@ -1990,6 +1999,8 @@
size_t tail;
int ret, found = 0;
bool eof_push = 0;
+ 
+    mutex_lock(&ldata->read_buf_lock);

/* N.B. avoid overrun if nr == 0 */
n = min(*nr, read_cnt(ldata));
@@ -2049,6 +2060,8 @@
ldata->line_start = ldata->read_tail;
tty_audit_push(tty);
}
+
+    mutex_unlock(&ldata->read_buf_lock);
return eof_push ? -EAGAIN : 0;
}