tty: xilinx_uartps: split sysrq handling

lockdep detects the following circular locking dependency:

CPU 0                      CPU 1
========================== ============================
cdns_uart_isr()            printk()
  uart_port_lock(port)       console_lock()
			     cdns_uart_console_write()
                               if (!port->sysrq)
                                 uart_port_lock(port)
  uart_handle_break()
    port->sysrq = ...
  uart_handle_sysrq_char()
    printk()
      console_lock()

The fixed commit attempts to avoid this situation by only taking the
port lock in cdns_uart_console_write if port->sysrq unset. However, if
(as shown above) cdns_uart_console_write runs before port->sysrq is set,
then it will try to take the port lock anyway. This may result in a
deadlock.

Fix this by splitting sysrq handling into two parts. We use the prepare
helper under the port lock and defer handling until we release the lock.

Fixes: 74ea66d4ca06 ("tty: xuartps: Improve sysrq handling")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Cc: stable@vger.kernel.org # c980248179d: serial: xilinx_uartps: Use port lock wrappers
Acked-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20250110213822.2107462-1-sean.anderson@linux.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
index beb151be4d328731aa37589288803cafbaf761db..92ec51870d1daf53c45d479336de3e0b13ae6f27 100644 (file)
--- a/drivers/tty/serial/xilinx_uartps.c
+++ b/drivers/tty/serial/xilinx_uartps.c
@@ -287,7 +287,7 @@ static void cdns_uart_handle_rx(void *dev_id, unsigned int isrstatus)
                                continue;
                }
 
-               if (uart_handle_sysrq_char(port, data))
+               if (uart_prepare_sysrq_char(port, data))
                        continue;
 
                if (is_rxbs_support) {
@@ -495,7 +495,7 @@ static irqreturn_t cdns_uart_isr(int irq, void *dev_id)
            !(readl(port->membase + CDNS_UART_CR) & CDNS_UART_CR_RX_DIS))
                cdns_uart_handle_rx(dev_id, isrstatus);
 
-       uart_port_unlock(port);
+       uart_unlock_and_check_sysrq(port);
        return IRQ_HANDLED;
 }
 
@@ -1380,9 +1380,7 @@ static void cdns_uart_console_write(struct console *co, const char *s,
        unsigned int imr, ctrl;
        int locked = 1;
 
-       if (port->sysrq)
-               locked = 0;
-       else if (oops_in_progress)
+       if (oops_in_progress)
                locked = uart_port_trylock_irqsave(port, &flags);
        else
                uart_port_lock_irqsave(port, &flags);