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>

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index f1d30f6..8ebd9f8 100644 (file)
--- 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 036cccd..1c3316a 100644 (file)
--- 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 */