[PATCH] n_hdlc fix read and write locking

[Paul Fulghum <paulkf@microgate.com>]

Fix locking in read and write code of n_hdlc line discipline.

2.6.36 replaced lock_kernel() with tty_lock().
The tty mutex is not dropped automatically when the thread
sleeps like the BKL. This results in a blocked read or write holding
the tty mutex and stalling operations by other devices that use
the tty mutex.

A review of n_hdlc read and write code shows:
1. neither BKL or tty mutex are required for correct operation
2. read can block while read data is available if data is posted
   between availability check and call to interruptible_sleep_on()
3. write does not set process state to TASK_INTERRUPTIBLE
   on each pass through the processing loop which can cause
   unneeded scheduling of the thread

The unnecessary tty mutex references have been removed.

Read changed to use same code as n_tty read
for completing reads and blocking.

Write corrected to set process state to
TASK_INTERRUPTIBLE on each pass through processing loop.

--- a/drivers/char/n_hdlc.c	2010-10-22 14:22:22.000000000 -0500
+++ b/drivers/char/n_hdlc.c	2010-10-25 12:36:12.000000000 -0500
@@ -581,8 +581,9 @@ static ssize_t n_hdlc_tty_read(struct tt
 			   __u8 __user *buf, size_t nr)
 {
 	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
-	int ret;
+	int ret = 0;
 	struct n_hdlc_buf *rbuf;
+	DECLARE_WAITQUEUE(wait, current);
 
 	if (debuglevel >= DEBUG_LEVEL_INFO)	
 		printk("%s(%d)n_hdlc_tty_read() called\n",__FILE__,__LINE__);
@@ -598,57 +599,55 @@ static ssize_t n_hdlc_tty_read(struct tt
 		return -EFAULT;
 	}
 
-	tty_lock();
+	add_wait_queue(&tty->read_wait, &wait);
 
 	for (;;) {
 		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
-			tty_unlock();
-			return -EIO;
+			ret = -EIO;
+			break;
 		}
+		if (tty_hung_up_p(file))
+			break;
 
-		n_hdlc = tty2n_hdlc (tty);
-		if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC ||
-			 tty != n_hdlc->tty) {
-			tty_unlock();
-			return 0;
-		}
+		set_current_state(TASK_INTERRUPTIBLE);
 
 		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
-		if (rbuf)
+		if (rbuf) {
+			if (rbuf->count > nr) {
+				/* too large for caller's buffer */
+				ret = -EOVERFLOW;
+			} else {
+				if (copy_to_user(buf, rbuf->buf, rbuf->count))
+					ret = -EFAULT;
+				else
+					ret = rbuf->count;
+			}
+
+			if (n_hdlc->rx_free_buf_list.count >
+			    DEFAULT_RX_BUF_COUNT)
+				kfree(rbuf);
+			else
+				n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, rbuf);
 			break;
+		}
 			
 		/* no data */
 		if (file->f_flags & O_NONBLOCK) {
-			tty_unlock();
-			return -EAGAIN;
+			ret = -EAGAIN;
+			break;
 		}
-			
-		interruptible_sleep_on (&tty->read_wait);
+
+		schedule();
+
 		if (signal_pending(current)) {
-			tty_unlock();
-			return -EINTR;
+			ret = -EINTR;
+			break;
 		}
 	}
-		
-	if (rbuf->count > nr)
-		/* frame too large for caller's buffer (discard frame) */
-		ret = -EOVERFLOW;
-	else {
-		/* Copy the data to the caller's buffer */
-		if (copy_to_user(buf, rbuf->buf, rbuf->count))
-			ret = -EFAULT;
-		else
-			ret = rbuf->count;
-	}
-	
-	/* return HDLC buffer to free list unless the free list */
-	/* count has exceeded the default value, in which case the */
-	/* buffer is freed back to the OS to conserve memory */
-	if (n_hdlc->rx_free_buf_list.count > DEFAULT_RX_BUF_COUNT)
-		kfree(rbuf);
-	else	
-		n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,rbuf);
-	tty_unlock();
+
+	remove_wait_queue(&tty->read_wait, &wait);
+	set_current_state(TASK_RUNNING);
+
 	return ret;
 	
 }	/* end of n_hdlc_tty_read() */
@@ -691,14 +690,15 @@ static ssize_t n_hdlc_tty_write(struct t
 		count = maxframe;
 	}
 	
-	tty_lock();
-
 	add_wait_queue(&tty->write_wait, &wait);
-	set_current_state(TASK_INTERRUPTIBLE);
+
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
 	
-	/* Allocate transmit buffer */
-	/* sleep until transmit buffer available */		
-	while (!(tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list))) {
+		tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
+		if (tbuf)
+			break;
+
 		if (file->f_flags & O_NONBLOCK) {
 			error = -EAGAIN;
 			break;
@@ -731,7 +731,7 @@ static ssize_t n_hdlc_tty_write(struct t
 		n_hdlc_buf_put(&n_hdlc->tx_buf_list,tbuf);
 		n_hdlc_send_frames(n_hdlc,tty);
 	}
-	tty_unlock();
+
 	return error;
 	
 }	/* end of n_hdlc_tty_write() */