Re: [PATCH v2 2/3] serial: sh-sci: Stop TX DMA workqueue during port shutdown

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Simon,

On Fri, Jul 13, 2018 at 9:42 AM Simon Horman <horms@verge.net.au> wrote:
> On Wed, Jul 11, 2018 at 05:15:57PM +0200, Geert Uytterhoeven wrote:
> > On Wed, Jul 11, 2018 at 4:52 PM Simon Horman <horms@verge.net.au> wrote:
> > > On Fri, Jul 06, 2018 at 11:05:42AM +0200, Geert Uytterhoeven wrote:
> > > > The transmit DMA workqueue is never stopped, hence the work function may
> > > > be called after the port has been shut down.
> > > >
> > > > Fix this race condition by cancelling queued work, if any, before DMA
> > > > release.  Don't initialize the work if DMA initialization failed, as it
> > > > won't be used anyway.
> > > >
> > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> >
> > > > --- a/drivers/tty/serial/sh-sci.c
> > > > +++ b/drivers/tty/serial/sh-sci.c
> > > > @@ -1293,6 +1293,7 @@ static void sci_tx_dma_release(struct sci_port *s)
> > > >  {
> > > >       struct dma_chan *chan = s->chan_tx_saved;
> > > >
> > > > +     cancel_work_sync(&s->work_tx);
> > > >       s->chan_tx_saved = s->chan_tx = NULL;
> > > >       s->cookie_tx = -EINVAL;
> > > >       dmaengine_terminate_all(chan);
> > > > @@ -1548,10 +1549,9 @@ static void sci_request_dma(struct uart_port *port)
> > > >                               __func__, UART_XMIT_SIZE,
> > > >                               port->state->xmit.buf, &s->tx_dma_addr);
> > > >
> > > > +                     INIT_WORK(&s->work_tx, work_fn_tx);
> > > >                       s->chan_tx_saved = s->chan_tx = chan;
> > >
> > > Is it ok that work_fn_tx reads and writes s->work_tx which
> >
> > writes s->chan_tx?
>
> Yes :)
>
> > > is set after the call to INIT_WORK() ?
> >
> > Yes, unlike interrupt handlers, it isn't called until software kicks the
> > workqueue using schedule_work(&s->work_tx);
>
> Ok, so we are sure access is only happening on one CPU?

sci_request_dma() is called during early port setup, before it can be used.
So no other CPU should be using it (unless it contains a time machine ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds