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Commit 5fbf1a65 authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
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Revert "tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc" 



This reverts commit 6a20dbd6.

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: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 501fed45

drivers/tty/tty_buffer.c

+2 −14
Original line number Diff line number Diff line
@@ -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;

include/linux/tty.h

+0 −1
Original line number Diff line number Diff line
@@ -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 */