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tty: Fix race condition if flushing tty flip buffers

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As Ilya Zykov identified in his patch 'PROBLEM: Race condition in
tty buffer's function flush_to_ldisc()', a race condition exists
which allows a parallel flush_to_ldisc() to flush and free the tty
flip buffers while those buffers are in-use. For example,

  CPU 0                         |  CPU 1                                  |  CPU 2
                                | flush_to_ldisc()                        |
                                |  grab spin lock                         |
tty_buffer_flush()              |                                         | flush_to_ldisc()
 wait for spin lock             |                                         |  wait for spin lock
                                |   if (!test_and_set_bit(TTYP_FLUSHING)) |
                                |    while (next flip buffer)             |
                                |     ...                                 |
                                |     drop spin lock                      |
  grab spin lock                |                                         |
   if (test_bit(TTYP_FLUSHING)) |                                         |
    set_bit(TTYP_FLUSHPENDING)  |      receive_buf()                      |
    drop spin lock              |                                         |
                                |                                         |   grab spin lock
                                |                                         |    if (!test_and_set_bit(TTYP_FLUSHING))
                                |                                         |    if (test_bit(TTYP_FLUSHPENDING))
                                |                                         |    __tty_buffer_flush()

CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is
transferring data from the head flip buffer.

The original patch was rejected under the assumption that parallel
flush_to_ldisc() was not possible. Because of necessary changes to
the workqueue api, work items can execute in parallel on SMP.

This patch differs slightly from the original patch by testing for
a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING
can only be set while the lock is dropped around receive_buf().

Reported-by: Ilya Zykov <linux@izyk.ru>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Ilya Zykov <linux@izyk.ru>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2013-03-20 13:20:43 -04:00 committed by Greg Kroah-Hartman
parent 31bdfc649f
commit 39f610e40e
1 changed files with 10 additions and 12 deletions
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22
drivers/tty/tty_buffer.c
View file

@ -449,11 +449,6 @@ static void flush_to_ldisc(struct work_struct *work)
tty_buffer_free(port, head);
continue;
}
/* Ldisc or user is trying to flush the buffers
we are feeding to the ldisc, stop feeding the
line discipline as we want to empty the queue */
if (test_bit(TTYP_FLUSHPENDING, &port->iflags))
break;
if (!tty->receive_room)
break;
if (count > tty->receive_room)
@ -465,17 +460,20 @@ static void flush_to_ldisc(struct work_struct *work)
disc->ops->receive_buf(tty, char_buf,
flag_buf, count);
spin_lock_irqsave(&buf->lock, flags);
/* 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
and empty the queue */
if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
__tty_buffer_flush(port);
clear_bit(TTYP_FLUSHPENDING, &port->iflags);
wake_up(&tty->read_wait);
break;
}
}
clear_bit(TTYP_FLUSHING, &port->iflags);
}
/* We may have a deferred request to flush the input buffer,
if so pull the chain under the lock and empty the queue */
if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
__tty_buffer_flush(port);
clear_bit(TTYP_FLUSHPENDING, &port->iflags);
wake_up(&tty->read_wait);
}
spin_unlock_irqrestore(&buf->lock, flags);
tty_ldisc_deref(disc);