1 files changed, 16 insertions, 15 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index f1d30f6945af..143deb62467d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -60,6 +60,7 @@ void tty_buffer_lock_exclusive(struct tty_port *port)
         atomic_inc(&buf->priority);
         mutex_lock(&buf->lock);
 }
+EXPORT_SYMBOL_GPL(tty_buffer_lock_exclusive);
 
 void tty_buffer_unlock_exclusive(struct tty_port *port)
 {
@@ -73,6 +74,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
         if (restart)
                 queue_work(system_unbound_wq, &buf->work);
 }
+EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);
 
 /**
  *      tty_buffer_space_avail  -       return unused buffer space
@@ -255,16 +257,15 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
         if (change || left < size) {
                 /* This is the slow path - looking for new buffers to use */
                 if ((n = tty_buffer_alloc(port, size)) != NULL) {
-                        unsigned long iflags;
-
                         n->flags = flags;
                         buf->tail = n;
-
-                        spin_lock_irqsave(&buf->flush_lock, iflags);
                         b->commit = b->used;
+                        /* paired w/ barrier in flush_to_ldisc(); ensures the
+                         * latest commit value can be read before the head is
+                         * advanced to the next buffer
+                         */
+                        smp_wmb();
                         b->next = n;
-                        spin_unlock_irqrestore(&buf->flush_lock, iflags);
-
                 } else if (change)
                         size = 0;
                 else
@@ -448,27 +449,28 @@ static void flush_to_ldisc(struct work_struct *work)
         mutex_lock(&buf->lock);
 
         while (1) {
-                unsigned long flags;
                 struct tty_buffer *head = buf->head;
+                struct tty_buffer *next;
                 int count;
 
                 /* Ldisc or user is trying to gain exclusive access */
                 if (atomic_read(&buf->priority))
                         break;
 
-                spin_lock_irqsave(&buf->flush_lock, flags);
+                next = head->next;
+                /* paired w/ barrier in __tty_buffer_request_room();
+                 * ensures commit value read is not stale if the head
+                 * is advancing to the next buffer
+                 */
+                smp_rmb();
                 count = head->commit - head->read;
                 if (!count) {
-                        if (head->next == NULL) {
-                                spin_unlock_irqrestore(&buf->flush_lock, flags);
+                        if (next == NULL)
                                 break;
-                        }
-                        buf->head = head->next;
-                        spin_unlock_irqrestore(&buf->flush_lock, flags);
+                        buf->head = next;
                         tty_buffer_free(port, head);
                         continue;
                 }
-                spin_unlock_irqrestore(&buf->flush_lock, flags);
 
                 count = receive_buf(tty, head, count);
                 if (!count)
@@ -523,7 +525,6 @@ void tty_buffer_init(struct tty_port *port)
         struct tty_bufhead *buf = &port->buf;
 
         mutex_init(&buf->lock);
-        spin_lock_init(&buf->flush_lock);
         tty_buffer_reset(&buf->sentinel, 0);
         buf->head = &buf->sentinel;
         buf->tail = &buf->sentinel;