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Message-ID: <20191204161330.GA28567@leoy-ThinkPad-X240s>
Date: Thu, 5 Dec 2019 00:13:30 +0800
From: Leo Yan <leo.yan@...aro.org>
To: Jeffrey Hugo <jeffrey.l.hugo@...il.com>
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 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);
}
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