[PATCH v3] serial: stm32: optimize spin lock usage

[dillon . minfei]

From: dillon min <dillon.minfei@xxxxxxxxx>

This patch aims to fix two potential bug:
- no lock to protect uart register in this case

  stm32_usart_threaded_interrupt()
     spin_lock(&port->lock);
     ...
     stm32_usart_receive_chars()
       uart_handle_sysrq_char();
       sysrq_function();
       printk();
         stm32_usart_console_write();
           locked = 0; //since port->sysrq is not zero,
                         no lock to protect forward register
                         access.

- if add spin_trylock_irqsave() to protect uart register for sysrq = 1 case,
  that might got recursive locking under UP.
  So, use uart_prepare_sysrq_char(), uart_unlock_and_check_sysrq()
  move sysrq handler position to irq/thread_d handler, just record
  sysrq_ch in stm32_usart_receive_chars() by uart_prepare_sysrq_char()
  delay the sysrq process to next interrupt handler.

  new flow:

  stm32_usart_threaded_interrupt()/stm32_usart_interrupt()
  spin_lock_irqsave(&port->lock);
  ...
  uart_unlock_and_check_sysrq();
     spin_unlock_irqrestore();
     handle_sysrq(sysrq_ch);
  stm32_usart_threaded_interrupt()//stm32_usart_interrupt() return

Cc: Johan Hovold <johan@xxxxxxxxxx>
Cc: Alexandre Torgue <alexandre.torgue@xxxxxxxxxxx>
Cc: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
Cc: Gerald Baeza <gerald.baeza@xxxxxxxxxxx>
Cc: Erwan Le Ray <erwan.leray@xxxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: dillon min <dillon.minfei@xxxxxxxxx>
---
v3: add uart_prepare_sysrq_char(), uart_unlock_and_check_sysrq() to move
    sysrq handler inside interrupt routinei to avoid recursive locking,
    according to Johan Hovold suggestion, thanks.

 drivers/tty/serial/stm32-usart.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index b3675cf25a69..981f50ec784e 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -271,7 +271,7 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
 			}
 		}
 
-		if (uart_handle_sysrq_char(port, c))
+		if (uart_prepare_sysrq_char(port, c))
 			continue;
 		uart_insert_char(port, sr, USART_SR_ORE, c, flag);
 	}
@@ -457,9 +457,10 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 	struct uart_port *port = ptr;
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
+	unsigned long flags;
 	u32 sr;
 
-	spin_lock(&port->lock);
+	spin_lock_irqsave(&port->lock, flags);
 
 	sr = readl_relaxed(port->membase + ofs->isr);
 
@@ -477,7 +478,7 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch))
 		stm32_usart_transmit_chars(port);
 
-	spin_unlock(&port->lock);
+	uart_unlock_and_check_sysrq(port, flags);
 
 	if (stm32_port->rx_ch)
 		return IRQ_WAKE_THREAD;
@@ -489,13 +490,14 @@ static irqreturn_t stm32_usart_threaded_interrupt(int irq, void *ptr)
 {
 	struct uart_port *port = ptr;
 	struct stm32_port *stm32_port = to_stm32_port(port);
+	unsigned long flags;
 
-	spin_lock(&port->lock);
+	spin_lock_irqsave(&port->lock, flags);
 
 	if (stm32_port->rx_ch)
 		stm32_usart_receive_chars(port, true);
 
-	spin_unlock(&port->lock);
+	uart_unlock_and_check_sysrq(port, flags);
 
 	return IRQ_HANDLED;
 }
@@ -1354,13 +1356,10 @@ static void stm32_usart_console_write(struct console *co, const char *s,
 	u32 old_cr1, new_cr1;
 	int locked = 1;
 
-	local_irq_save(flags);
-	if (port->sysrq)
-		locked = 0;
-	else if (oops_in_progress)
-		locked = spin_trylock(&port->lock);
+	if (oops_in_progress)
+		locked = spin_trylock_irqsave(&port->lock, flags);
 	else
-		spin_lock(&port->lock);
+		spin_lock_irqsave(&port->lock, flags);
 
 	/* Save and disable interrupts, enable the transmitter */
 	old_cr1 = readl_relaxed(port->membase + ofs->cr1);
@@ -1374,8 +1373,7 @@ static void stm32_usart_console_write(struct console *co, const char *s,
 	writel_relaxed(old_cr1, port->membase + ofs->cr1);
 
 	if (locked)
-		spin_unlock(&port->lock);
-	local_irq_restore(flags);
+		spin_unlock_irqrestore(&port->lock, flags);
 }
 
 static int stm32_usart_console_setup(struct console *co, char *options)
-- 
2.7.4