Re: [PATCH] tty: serial: fsl_lpuart: fix race on RX DMA shutdown

["Sverdlin, Alexander" <alexander.sverdlin@siemens.com>]

Hello Jiri,

On Mon, 2023-03-06 at 10:43 +0100, Jiri Slaby wrote:
> On 06. 03. 23, 10:00, A. Sverdlin wrote:
> > From: Alexander Sverdlin <alexander.sverdlin@siemens.com>
> > 
> >  From time to time DMA completion can come in the middle of DMA
> > shutdown:
> > 
> > <process ctx>:                          <IRQ>:
> > lpuart32_shutdown()
> >    lpuart_dma_shutdown()
> >      del_timer_sync()
> >                                         lpuart_dma_rx_complete()
> >                                           lpuart_copy_rx_to_tty()
> >                                             mod_timer()
> >      lpuart_dma_rx_free()
> > 
> > When the timer fires a bit later, sport->dma_rx_desc is NULL:
> > 
> > Unable to handle kernel NULL pointer dereference at virtual address
> > 0000000000000004
> > pc : lpuart_copy_rx_to_tty+0xcc/0x5bc
> > lr : lpuart_timer_func+0x1c/0x2c
> > Call trace:
> >   lpuart_copy_rx_to_tty
> >   lpuart_timer_func
> >   call_timer_fn
> >   __run_timers.part.0
> >   run_timer_softirq
> >   __do_softirq
> >   __irq_exit_rcu
> >   irq_exit
> >   handle_domain_irq
> >   gic_handle_irq
> >   call_on_irq_stack
> >   do_interrupt_handler
> >   ...
> > 
> > To fix this fold del_timer_sync() into lpuart_dma_rx_free() after
> > dmaengine_terminate_sync() to make sure timer will not be re-
> > started in
> > lpuart_copy_rx_to_tty() <= lpuart_dma_rx_complete().
> > 
> > Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
> 
> This should have some Fixes: tag, I believe.
> 

sure, you are right, I'll use
Fixes: 4a8588a1cf86 ("serial: fsl_lpuart: delete timer on shutdown")

on respin.

> > ---
> >   drivers/tty/serial/fsl_lpuart.c | 11 +++--------
> >   1 file changed, 3 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > b/drivers/tty/serial/fsl_lpuart.c
> > index e945f41b93d43..47c267ee22e04 100644
> > --- a/drivers/tty/serial/fsl_lpuart.c
> > +++ b/drivers/tty/serial/fsl_lpuart.c
> > @@ -1354,6 +1354,7 @@ static void lpuart_dma_rx_free(struct
> > uart_port *port)
> >         struct dma_chan *chan = sport->dma_rx_chan;
> >   
> >         dmaengine_terminate_sync(chan);
> 
> Maybe I'm missing something (I haven't looked into the code), but
> what 
> happens if the timer ticks here? Won't the dmaengine be restarted
> there?
> 

It's cyclic (used dmaengine_prep_dma_cyclic()), I see no restart in
lpuart_copy_rx_to_tty(), but maybe someone could double-check.

> > +       del_timer_sync(&sport->lpuart_timer);
> >         dma_unmap_sg(chan->device->dev, &sport->rx_sgl, 1,
> > DMA_FROM_DEVICE);
> >         kfree(sport->rx_ring.buf);
> >         sport->rx_ring.tail = 0;
> 
> 

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com