[ +cc Paul Mackerras, linux-ppp, netdev ]
I neglected to cc the proper folks. Sorry about that.
Regards,
Peter Hurley
On Mon, 2013-03-11 at 16:44 -0400, Peter Hurley wrote:
> tty_perform_flush() can deadlock when called while holding
> a line discipline reference. By definition, all ldisc drivers
> hold a ldisc reference, so calls originating from ldisc drivers
> must not block for a ldisc reference.
>
> The deadlock can occur when:
> CPU 0 | CPU 1
> |
> tty_ldisc_ref(tty) |
> .... | <line discipline halted>
> tty_ldisc_ref_wait(tty) |
> |
>
> CPU 0 cannot progess because it cannot obtain an ldisc reference
> with the line discipline has been halted (thus no new references
> are granted).
> CPU 1 cannot progress because an outstanding ldisc reference
> has not been released.
>
> An in-tree call-tree audit of tty_perform_flush() [1] shows 5
> ldisc drivers calling tty_perform_flush() indirectly via
> n_tty_ioctl_helper() and 2 ldisc drivers calling directly.
> A single tty driver safely uses the function.
>
> [1]
> Recursive usage:
>
> /* These functions are line discipline ioctls and thus
> * recursive wrt line discipline references */
>
> tty_perform_flush() - ./drivers/tty/tty_ioctl.c
> n_tty_ioctl_helper()
> hci_uart_tty_ioctl(default) - drivers/bluetooth/hci_ldisc.c (N_HCI)
> n_hdlc_tty_ioctl(default) - drivers/tty/n_hdlc.c (N_HDLC)
> gsmld_ioctl(default) - drivers/tty/n_gsm.c (N_GSM0710)
> n_tty_ioctl(default) - drivers/tty/n_tty.c (N_TTY)
> gigaset_tty_ioctl(default) - drivers/isdn/gigaset/ser-gigaset.c (N_GIGASET_M101)
> ppp_synctty_ioctl(TCFLSH) - drivers/net/ppp/pps_synctty.c
> ppp_asynctty_ioctl(TCFLSH) - drivers/net/ppp/ppp_async.c
>
> Non-recursive use:
>
> tty_perform_flush() - drivers/tty/tty_ioctl.c
> ipw_ioctl(TCFLSH) - drivers/tty/ipwireless/tty.c
> /* This function is a tty i/o ioctl method, which
> * is invoked by tty_ioctl() */
>
> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/net/ppp/ppp_async.c | 2 +-
> drivers/net/ppp/ppp_synctty.c | 2 +-
> drivers/tty/tty_ioctl.c | 28 +++++++++++++++++++---------
> 3 files changed, 21 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
> index a031f6b..9c889e0 100644
> --- a/drivers/net/ppp/ppp_async.c
> +++ b/drivers/net/ppp/ppp_async.c
> @@ -314,7 +314,7 @@ ppp_asynctty_ioctl(struct tty_struct *tty, struct file *file,
> /* flush our buffers and the serial port's buffer */
> if (arg == TCIOFLUSH || arg == TCOFLUSH)
> ppp_async_flush_output(ap);
> - err = tty_perform_flush(tty, arg);
> + err = n_tty_ioctl_helper(tty, file, cmd, arg);
> break;
>
> case FIONREAD:
> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
> index 1a12033..bdf3b13 100644
> --- a/drivers/net/ppp/ppp_synctty.c
> +++ b/drivers/net/ppp/ppp_synctty.c
> @@ -355,7 +355,7 @@ ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
> /* flush our buffers and the serial port's buffer */
> if (arg == TCIOFLUSH || arg == TCOFLUSH)
> ppp_sync_flush_output(ap);
> - err = tty_perform_flush(tty, arg);
> + err = n_tty_ioctl_helper(tty, file, cmd, arg);
> break;
>
> case FIONREAD:
> diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
> index d58b92c..935b032 100644
> --- a/drivers/tty/tty_ioctl.c
> +++ b/drivers/tty/tty_ioctl.c
> @@ -1086,14 +1086,12 @@ int tty_mode_ioctl(struct tty_struct *tty, struct file *file,
> }
> EXPORT_SYMBOL_GPL(tty_mode_ioctl);
>
> -int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
> +
> +/* Caller guarantees ldisc reference is held */
> +static int __tty_perform_flush(struct tty_struct *tty, unsigned long arg)
> {
> - struct tty_ldisc *ld;
> - int retval = tty_check_change(tty);
> - if (retval)
> - return retval;
> + struct tty_ldisc *ld = tty->ldisc;
>
> - ld = tty_ldisc_ref_wait(tty);
> switch (arg) {
> case TCIFLUSH:
> if (ld && ld->ops->flush_buffer) {
> @@ -1111,12 +1109,24 @@ int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
> tty_driver_flush_buffer(tty);
> break;
> default:
> - tty_ldisc_deref(ld);
> return -EINVAL;
> }
> - tty_ldisc_deref(ld);
> return 0;
> }
> +
> +int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
> +{
> + struct tty_ldisc *ld;
> + int retval = tty_check_change(tty);
> + if (retval)
> + return retval;
> +
> + ld = tty_ldisc_ref_wait(tty);
> + retval = __tty_perform_flush(tty, arg);
> + if (ld)
> + tty_ldisc_deref(ld);
> + return retval;
> +}
> EXPORT_SYMBOL_GPL(tty_perform_flush);
>
> int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
> @@ -1155,7 +1165,7 @@ int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
> }
> return 0;
> case TCFLSH:
> - return tty_perform_flush(tty, arg);
> + return __tty_perform_flush(tty, arg);
> default:
> /* Try the mode commands */
> return tty_mode_ioctl(tty, file, cmd, arg);
--
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