Re: [PATCH] tty: fix data race on tty_buffer.commit

[Peter Hurley]

On Tue, Sep 8, 2015 at 9:03 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> Race on buffer data happens when newly committed data is
> picked up by an old flush work in the following scenario:
> __tty_buffer_request_room does a plain write of tail->commit,
> no barriers were executed before that.
> At this point flush_to_ldisc reads this new value of commit,
> and reads buffer data, no barriers in between.
> The committed buffer data is not necessary visible to flush_to_ldisc.
>
> Similar bug happens when tty_schedule_flip commits data.
>
> Update commit with smp_store_release and read commit with
> smp_load_acquire, as it is commit that signals data readiness.
> This is orthogonal to the existing synchronization on tty_buffer.next,
> which is required to not dismiss a buffer with unconsumed data.
>
> The data race was found with KernelThreadSanitizer (KTSAN).

Looks good.

Same comments as the other patch; needs changelog revision
information.

Thanks for your work on this; really good stuff.

Regards,
Peter Hurley

> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---
>  drivers/tty/tty_buffer.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 5a3fa89..7ed2006 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -290,7 +290,10 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
>                 if (n != NULL) {
>                         n->flags = flags;
>                         buf->tail = n;
> -                       b->commit = b->used;
> +                       /* paired w/ acquire in flush_to_ldisc(); ensures
> +                        * flush_to_ldisc() sees buffer data.
> +                        */
> +                       smp_store_release(&b->commit, b->used);
>                         /* paired w/ acquire in flush_to_ldisc(); ensures the
>                          * latest commit value can be read before the head is
>                          * advanced to the next buffer
> @@ -393,7 +396,10 @@ void tty_schedule_flip(struct tty_port *port)
>  {
>         struct tty_bufhead *buf = &port->buf;
>
> -       buf->tail->commit = buf->tail->used;
> +       /* paired w/ acquire in flush_to_ldisc(); ensures
> +        * flush_to_ldisc() sees buffer data.
> +        */
> +       smp_store_release(&buf->tail->commit, buf->tail->used);
>         schedule_work(&buf->work);
>  }
>  EXPORT_SYMBOL(tty_schedule_flip);
> @@ -491,7 +497,10 @@ static void flush_to_ldisc(struct work_struct *work)
>                  * is advancing to the next buffer
>                  */
>                 next = smp_load_acquire(&head->next);
> -               count = head->commit - head->read;
> +               /* paired w/ release in __tty_buffer_request_room() or in
> +                * tty_buffer_flush(); ensures we see the committed buffer data
> +                */
> +               count = smp_load_acquire(&head->commit) - head->read;
>                 if (!count) {
>                         if (next == NULL) {
>                                 check_other_closed(tty);
> --
> 2.5.0.457.gab17608
>
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