From: dillon min <redacted>

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@kernel.org>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Gerald Baeza <redacted>
Cc: Erwan Le Ray <redacted>
Reported-by: kernel test robot <redacted>
Signed-off-by: dillon min <redacted>
---
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);