Re: [PATCH v2 2/2] tty: serial: msm_serial: Fix deadlock caused by recursive output

[Jeffrey Hugo]

On Wed, Dec 4, 2019 at 9:13 AM Leo Yan <leo.yan@xxxxxxxxxx> wrote:
>
> On Tue, Dec 03, 2019 at 03:42:31PM -0700, Jeffrey Hugo wrote:
>
> [...]
>
> > > > > This patch fixes the deadlock issue for recursive output; it adds a
> > > > > variable 'curr_user' to indicate the uart port is used by which CPU, if
> > > > > the CPU has acquired spinlock and wants to execute recursive output,
> > > > > it will directly bail out.  Here we don't choose to avoid locking and
> > > > > print out log, the reason is in this case we don't want to reset the
> > > > > uart port with function msm_reset_dm_count(); otherwise it can introduce
> > > > > confliction with other flows and results in uart port malfunction and
> > > > > later cannot output anymore.
> > > >
> > > > Is this not fixable?  Sure, fixing the deadlock is an improvement, but
> > > > dropping logs (particularly a memory warning like in your example)
> > > > seems undesirable.
> > >
> > > Thanks a lot for your reviewing, Jeffrey.
> > >
> > > Agreed with you for the concern.
> > >
> > > To be honest, I am not familiar with the msm uart driver, so have no
> > > confidence which is the best way for uart port operations.  I can
> > > think out one possible fixing is shown in below, if detects the lock
> > > is not acquired then it will force to reset UART port before exit the
> > > function __msm_console_write().
> > >
> > > This approach is not tested yet and it looks too arbitrary; I will
> > > give a try for it.  At the meantime, welcome any insight suggestion
> > > with proper register operations.
> >
> > According to the documentation, NCF_TX is only needed for SW transmit
> > mode, where software is directly puttting characters in the fifo.  Its
> > not needed for BAM mode.  According to your example, recursive console
> > printing will only happen in BAM mode, and not in SW mode.  Perhaps if
> > we put the NCF_TX uses to just the SW mode, we avoid the issue and can
> > allow recursive printing?
>
> Thanks for the suggestion!  But based on the suggestion, I tried to
> change code as below, the console even cannot work when boot the
> kernel:
>
>  static void msm_reset_dm_count(struct uart_port *port, int count)
>  {
> +       u32 val;
> +
>         msm_wait_for_xmitr(port);
> -       msm_write(port, count, UARTDM_NCF_TX);
> -       msm_read(port, UARTDM_NCF_TX);
> +
> +       val = msm_read(port, UARTDM_DMEN);
> +
> +       /*
> +        * NCF is only enabled for SW transmit mode and is
> +        * skipped for BAM mode.
> +        */
> +       if (!(val & UARTDM_DMEN_TX_BAM_ENABLE) &&
> +           !(val & UARTDM_DMEN_RX_BAM_ENABLE)) {
> +               msm_write(port, count, UARTDM_NCF_TX);
> +               msm_read(port, UARTDM_NCF_TX);
> +       }
>  }
>
>
> Alternatively, when exit from __msm_console_write() and if detect the
> case for without acquiring spinlock, invoke msm_wait_for_xmitr() to wait
> for transmit completion looks a good candidate solution. The updated
> patch is as below.  Please let me know if this is doable?
>
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 1db79ee8a886..aa6a494c898d 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -190,6 +190,7 @@ struct msm_port {
>         bool                    break_detected;
>         struct msm_dma          tx_dma;
>         struct msm_dma          rx_dma;
> +       struct cpumask          curr_user;
>  };
>
>  #define UART_TO_MSM(uart_port) container_of(uart_port, struct msm_port, uart)
> @@ -440,6 +441,7 @@ static void msm_complete_tx_dma(void *args)
>         u32 val;
>
>         spin_lock_irqsave(&port->lock, flags);
> +       cpumask_set_cpu(smp_processor_id(), &msm_port->curr_user);
>
>         /* Already stopped */
>         if (!dma->count)
> @@ -474,6 +476,7 @@ static void msm_complete_tx_dma(void *args)
>
>         msm_handle_tx(port);
>  done:
> +       cpumask_clear_cpu(smp_processor_id(), &msm_port->curr_user);
>         spin_unlock_irqrestore(&port->lock, flags);
>  }
>
> @@ -548,6 +551,7 @@ static void msm_complete_rx_dma(void *args)
>         u32 val;
>
>         spin_lock_irqsave(&port->lock, flags);
> +       cpumask_set_cpu(smp_processor_id(), &msm_port->curr_user);
>
>         /* Already stopped */
>         if (!dma->count)
> @@ -594,6 +598,7 @@ static void msm_complete_rx_dma(void *args)
>
>         msm_start_rx_dma(msm_port);
>  done:
> +       cpumask_clear_cpu(smp_processor_id(), &msm_port->curr_user);
>         spin_unlock_irqrestore(&port->lock, flags);
>
>         if (count)
> @@ -932,6 +937,7 @@ static irqreturn_t msm_uart_irq(int irq, void *dev_id)
>         u32 val;
>
>         spin_lock_irqsave(&port->lock, flags);
> +       cpumask_set_cpu(smp_processor_id(), &msm_port->curr_user);
>         misr = msm_read(port, UART_MISR);
>         msm_write(port, 0, UART_IMR); /* disable interrupt */
>
> @@ -963,6 +969,7 @@ static irqreturn_t msm_uart_irq(int irq, void *dev_id)
>                 msm_handle_delta_cts(port);
>
>         msm_write(port, msm_port->imr, UART_IMR); /* restore interrupt */
> +       cpumask_clear_cpu(smp_processor_id(), &msm_port->curr_user);
>         spin_unlock_irqrestore(&port->lock, flags);
>
>         return IRQ_HANDLED;
> @@ -1573,10 +1580,12 @@ static inline struct uart_port *msm_get_port_from_line(unsigned int line)
>  static void __msm_console_write(struct uart_port *port, const char *s,
>                                 unsigned int count, bool is_uartdm)
>  {
> +       struct msm_port *msm_port = UART_TO_MSM(port);
>         int i;
>         int num_newlines = 0;
>         bool replaced = false;
>         void __iomem *tf;
> +       int locked = 1;
>
>         if (is_uartdm)
>                 tf = port->membase + UARTDM_TF;
> @@ -1589,7 +1598,15 @@ static void __msm_console_write(struct uart_port *port, const char *s,
>                         num_newlines++;
>         count += num_newlines;
>
> -       spin_lock(&port->lock);
> +       if (port->sysrq)
> +               locked = 0;
> +       else if (oops_in_progress)
> +               locked = spin_trylock(&port->lock);
> +       else if (cpumask_test_cpu(smp_processor_id(), &msm_port->curr_user))
> +               locked = 0;
> +       else
> +               spin_lock(&port->lock);
> +
>         if (is_uartdm)
>                 msm_reset_dm_count(port, count);
>
> @@ -1625,7 +1642,12 @@ static void __msm_console_write(struct uart_port *port, const char *s,
>                 iowrite32_rep(tf, buf, 1);
>                 i += num_chars;
>         }
> -       spin_unlock(&port->lock);
> +
> +       if (!locked)
> +               msm_wait_for_xmitr(port);

Sorry, catching up from some travel.

I don't understand this.  At this point, haven't we already called
msm_reset_dm_count() and "corrupted" the state of the hardware?

> +
> +       if (locked)
> +               spin_unlock(&port->lock);
>  }