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

[Leo Yan <leo.yan@linaro.org>]

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);
 }