tty: Ensure single-threaded flip buffer consumer with mutex

The buffer work may race with parallel tty_buffer_flush. Use a
mutex to guarantee exclusive modify access to the head flip
buffer.

Remove the unneeded spin lock.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 21 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 685757c6ce87..c3c606c52722 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -157,8 +157,6 @@ static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
  *      flush all the buffers containing receive data. Caller must
  *      hold the buffer lock and must have ensured no parallel flush to
  *      ldisc is running.
- *
- *      Locking: Caller must hold tty->buf.lock
  */
 
 static void __tty_buffer_flush(struct tty_port *port)
@@ -182,29 +180,29 @@ static void __tty_buffer_flush(struct tty_port *port)
  *      being processed by flush_to_ldisc then we defer the processing
  *      to that function
  *
- *      Locking: none
+ *      Locking: takes flush_mutex to ensure single-threaded flip buffer
+ *               'consumer'
  */
 
 void tty_buffer_flush(struct tty_struct *tty)
 {
         struct tty_port *port = tty->port;
         struct tty_bufhead *buf = &port->buf;
-        unsigned long flags;
-
-        spin_lock_irqsave(&buf->lock, flags);
 
+        mutex_lock(&buf->flush_mutex);
         /* If the data is being pushed to the tty layer then we can't
            process it here. Instead set a flag and the flush_to_ldisc
            path will process the flush request before it exits */
         if (test_bit(TTYP_FLUSHING, &port->iflags)) {
                 set_bit(TTYP_FLUSHPENDING, &port->iflags);
-                spin_unlock_irqrestore(&buf->lock, flags);
+                mutex_unlock(&buf->flush_mutex);
                 wait_event(tty->read_wait,
                                 test_bit(TTYP_FLUSHPENDING, &port->iflags) == 0);
                 return;
-        } else
-                __tty_buffer_flush(port);
-        spin_unlock_irqrestore(&buf->lock, flags);
+        }
+
+        __tty_buffer_flush(port);
+        mutex_unlock(&buf->flush_mutex);
 }
 
 /**
@@ -408,9 +406,10 @@ receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
  *      This routine is called out of the software interrupt to flush data
  *      from the buffer chain to the line discipline.
  *
- *      Locking: holds tty->buf.lock to guard buffer list. Drops the lock
- *      while invoking the line discipline receive_buf method. The
- *      receive_buf method is single threaded for each tty instance.
+ *      The receive_buf method is single threaded for each tty instance.
+ *
+ *      Locking: takes flush_mutex to ensure single-threaded flip buffer
+ *               'consumer'
  */
 
 static void flush_to_ldisc(struct work_struct *work)
@@ -418,7 +417,6 @@ static void flush_to_ldisc(struct work_struct *work)
         struct tty_port *port = container_of(work, struct tty_port, buf.work);
         struct tty_bufhead *buf = &port->buf;
         struct tty_struct *tty;
-        unsigned long   flags;
         struct tty_ldisc *disc;
 
         tty = port->itty;
@@ -429,7 +427,7 @@ static void flush_to_ldisc(struct work_struct *work)
         if (disc == NULL)
                 return;
 
-        spin_lock_irqsave(&buf->lock, flags);
+        mutex_lock(&buf->flush_mutex);
 
         if (!test_and_set_bit(TTYP_FLUSHING, &port->iflags)) {
                 while (1) {
@@ -444,11 +442,13 @@ static void flush_to_ldisc(struct work_struct *work)
                                 tty_buffer_free(port, head);
                                 continue;
                         }
-                        spin_unlock_irqrestore(&buf->lock, flags);
+
+                        mutex_unlock(&buf->flush_mutex);
 
                         count = receive_buf(tty, head, count);
 
-                        spin_lock_irqsave(&buf->lock, flags);
+                        mutex_lock(&buf->flush_mutex);
+
                         /* Ldisc or user is trying to flush the buffers.
                            We may have a deferred request to flush the
                            input buffer, if so pull the chain under the lock
@@ -464,7 +464,7 @@ static void flush_to_ldisc(struct work_struct *work)
                 clear_bit(TTYP_FLUSHING, &port->iflags);
         }
 
-        spin_unlock_irqrestore(&buf->lock, flags);
+        mutex_unlock(&buf->flush_mutex);
 
         tty_ldisc_deref(disc);
 }
@@ -514,15 +514,13 @@ EXPORT_SYMBOL(tty_flip_buffer_push);
  *
  *      Set up the initial state of the buffer management for a tty device.
  *      Must be called before the other tty buffer functions are used.
- *
- *      Locking: none
  */
 
 void tty_buffer_init(struct tty_port *port)
 {
         struct tty_bufhead *buf = &port->buf;
 
-        spin_lock_init(&buf->lock);
+        mutex_init(&buf->flush_mutex);
         tty_buffer_reset(&buf->sentinel, 0);
         buf->head = &buf->sentinel;
         buf->tail = &buf->sentinel;