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

[Jeffrey Hugo]

On Wed, Dec 4, 2019 at 9:21 AM Rainer Sickinger
<rainersickinger.official@xxxxxxxxx> wrote:
>
> Can't you just exit with System.exit()?

Isn't System.exit() a Java thing, and we are in a C environment?

>
> Am Mi., 4. Dez. 2019 um 17:14 Uhr schrieb Leo Yan <leo.yan@xxxxxxxxxx>:
>>
>> 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);
>> +
>> +       if (locked)
>> +               spin_unlock(&port->lock);
>>  }