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Date:   Thu, 3 Jan 2019 00:04:02 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Paul Fulghum <paulkf@...rogate.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Alan Cox <alan@...rguk.ukuu.org.uk>,
        gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/n_hdlc: fix sleep in !TASK_RUNNING state warning

On 2019/01/01 12:11, Paul Fulghum wrote:
> NAK to this patch. It causes lost wakeups in both read and write paths.
> 
> The write path does not need changing.
> 
> The read path can be fixed by setting current to TASK_RUNNING at the top of the if (rbuf) block
> so the warning is not triggered by copy_to_user(). If this block runs the condition is satisfied
> and it breaks out of the polling loop where it is already being set to TASK_RUNNING and removed
> from the wait queue. This particular path just needs to account for the copy_to_user which occurs
> before breaking out.
> 
> I’ll make a patch to do this when I have the ability to test it in a day or two.
> 

OK. Then, any chance it is rewritten using wait_event_interruptible() in order to reduce lines?
( wait_event_interruptible() automatically calls might_sleep(), but is it acceptable for you? )

---
 drivers/tty/n_hdlc.c | 126 ++++++++++++---------------------------------------
 1 file changed, 30 insertions(+), 96 deletions(-)

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 8223d02..2e4ccf9 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -562,8 +562,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 {
 	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
 	int ret = 0;
-	struct n_hdlc_buf *rbuf;
-	DECLARE_WAITQUEUE(wait, current);
+	struct n_hdlc_buf *rbuf = NULL;
 
 	if (debuglevel >= DEBUG_LEVEL_INFO)	
 		printk("%s(%d)n_hdlc_tty_read() called\n",__FILE__,__LINE__);
@@ -579,58 +578,26 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 		return -EFAULT;
 	}
 
-	add_wait_queue(&tty->read_wait, &wait);
-
-	for (;;) {
-		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
-			ret = -EIO;
-			break;
-		}
-		if (tty_hung_up_p(file))
-			break;
-
-		set_current_state(TASK_INTERRUPTIBLE);
-
-		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
-		if (rbuf) {
-			if (rbuf->count > nr) {
-				/* too large for caller's buffer */
-				ret = -EOVERFLOW;
-			} else {
-				__set_current_state(TASK_RUNNING);
-				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 (tty_io_nonblock(tty, file)) {
-			ret = -EAGAIN;
-			break;
-		}
-
-		schedule();
-
-		if (signal_pending(current)) {
-			ret = -EINTR;
-			break;
-		}
+	if (wait_event_interruptible(tty->read_wait,
+	     (ret = -EIO, test_bit(TTY_OTHER_CLOSED, &tty->flags)) ||
+	     (ret = 0, tty_hung_up_p(file)) ||
+	     (rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list)) != NULL ||
+	     (ret = -EAGAIN, tty_io_nonblock(tty, file))))
+		return -EINTR;
+	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);
 	}
-
-	remove_wait_queue(&tty->read_wait, &wait);
-	__set_current_state(TASK_RUNNING);
-
 	return ret;
-	
 }	/* end of n_hdlc_tty_read() */
 
 /**
@@ -645,21 +612,13 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
 			    const unsigned char *data, size_t count)
 {
-	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
+	struct n_hdlc *n_hdlc;
 	int error = 0;
-	DECLARE_WAITQUEUE(wait, current);
-	struct n_hdlc_buf *tbuf;
+	struct n_hdlc_buf *tbuf = NULL;
 
 	if (debuglevel >= DEBUG_LEVEL_INFO)	
 		printk("%s(%d)n_hdlc_tty_write() called count=%zd\n",
 			__FILE__,__LINE__,count);
-		
-	/* Verify pointers */
-	if (!n_hdlc)
-		return -EIO;
-
-	if (n_hdlc->magic != HDLC_MAGIC)
-		return -EIO;
 
 	/* verify frame size */
 	if (count > maxframe ) {
@@ -670,40 +629,14 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
 				maxframe );
 		count = maxframe;
 	}
-	
-	add_wait_queue(&tty->write_wait, &wait);
 
-	for (;;) {
-		set_current_state(TASK_INTERRUPTIBLE);
-	
-		tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
-		if (tbuf)
-			break;
-
-		if (tty_io_nonblock(tty, file)) {
-			error = -EAGAIN;
-			break;
-		}
-		schedule();
-			
-		n_hdlc = tty2n_hdlc (tty);
-		if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC || 
-		    tty != n_hdlc->tty) {
-			printk("n_hdlc_tty_write: %p invalid after wait!\n", n_hdlc);
-			error = -EIO;
-			break;
-		}
-			
-		if (signal_pending(current)) {
-			error = -EINTR;
-			break;
-		}
-	}
-
-	__set_current_state(TASK_RUNNING);
-	remove_wait_queue(&tty->write_wait, &wait);
-
-	if (!error) {		
+	if (wait_event_interruptible(tty->write_wait,
+	     (error = -EIO, n_hdlc = tty2n_hdlc(tty), /* Verify pointers */
+	      !n_hdlc || n_hdlc->magic != HDLC_MAGIC || tty != n_hdlc->tty) ||
+	     (tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list)) != NULL ||
+	     (error = -EAGAIN, tty_io_nonblock(tty, file))))
+		return -EINTR;
+	if (tbuf) {
 		/* Retrieve the user's buffer */
 		memcpy(tbuf->buf, data, count);
 
@@ -711,8 +644,9 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
 		tbuf->count = error = count;
 		n_hdlc_buf_put(&n_hdlc->tx_buf_list,tbuf);
 		n_hdlc_send_frames(n_hdlc,tty);
+	} else if (error == -EIO) {
+		printk("n_hdlc_tty_write: %p invalid!\n", n_hdlc);
 	}
-
 	return error;
 	
 }	/* end of n_hdlc_tty_write() */
-- 
1.8.3.1

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