Revert "tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc"

This reverts commit 6a20dbd6caa2358716136144bf524331d70b1e03.

Although the commit correctly identifies an unsafe race condition
between __tty_buffer_request_room() and flush_to_ldisc(), the commit
fixes the race with an unnecessary spinlock in a lockless algorithm.

The follow-on commit, "tty: Fix lockless tty buffer race" fixes
the race locklessly.

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

2 files changed, 2 insertions, 15 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index f1d30f6945af..8ebd9f88a6f6 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -255,16 +255,11 @@ 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;
+                        smp_mb();
                         b->next = n;
-                        spin_unlock_irqrestore(&buf->flush_lock, iflags);
-
                 } else if (change)
                         size = 0;
                 else
@@ -448,7 +443,6 @@ static void flush_to_ldisc(struct work_struct *work)
         mutex_lock(&buf->lock);
 
         while (1) {
-                unsigned long flags;
                 struct tty_buffer *head = buf->head;
                 int count;
 
@@ -456,19 +450,14 @@ static void flush_to_ldisc(struct work_struct *work)
                 if (atomic_read(&buf->priority))
                         break;
 
-                spin_lock_irqsave(&buf->flush_lock, flags);
                 count = head->commit - head->read;
                 if (!count) {
-                        if (head->next == NULL) {
-                                spin_unlock_irqrestore(&buf->flush_lock, flags);
+                        if (head->next == NULL)
                                 break;
-                        }
                         buf->head = head->next;
-                        spin_unlock_irqrestore(&buf->flush_lock, flags);
                         tty_buffer_free(port, head);
                         continue;
                 }
-                spin_unlock_irqrestore(&buf->flush_lock, flags);
 
                 count = receive_buf(tty, head, count);
                 if (!count)
@@ -523,7 +512,6 @@ 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;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 036cccd80d9f..1c3316a47d7e 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -61,7 +61,6 @@ struct tty_bufhead {
         struct tty_buffer *head;        /* Queue head */
         struct work_struct work;
         struct mutex       lock;
-        spinlock_t         flush_lock;
         atomic_t           priority;
         struct tty_buffer sentinel;
         struct llist_head free;         /* Free queue head */