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Date: Mon, 16 Dec 2019 11:49:52 -0700
From: Jeffrey Hugo <jeffrey.l.hugo@...il.com>
To: Leo Yan <leo.yan@...aro.org>
Cc: Andy Gross <agross@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>,
Bjorn Andersson <bjorn.andersson@...aro.org>,
Stephen Boyd <swboyd@...omium.org>,
Nicolas Dechesne <nicolas.dechesne@...aro.org>,
MSM <linux-arm-msm@...r.kernel.org>,
linux-serial@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] tty: serial: msm_serial: Fix deadlock caused by
recursive output
On Wed, Dec 4, 2019 at 9:13 AM Leo Yan <leo.yan@...aro.org> 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);
> }
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