linux-kernel - [PATCH] tty: xilinx_uartps: split sysrq handling

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Message-Id: <20250110213822.2107462-1-sean.anderson@linux.dev>
Date: Fri, 10 Jan 2025 16:38:22 -0500
From: Sean Anderson <sean.anderson@...ux.dev>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	linux-serial@...r.kernel.org
Cc: Manikanta Guntupalli <manikanta.guntupalli@....com>,
	linux-arm-kernel@...ts.infradead.org,
	John Ogness <john.ogness@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Michal Simek <michal.simek@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sean Anderson <sean.anderson@...ux.dev>,
	stable@...r.kernel.org
Subject: [PATCH] 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@...ux.dev>
Cc: <stable@...r.kernel.org> # c980248179d: serial: xilinx_uartps: Use port lock wrappers
---

 drivers/tty/serial/xilinx_uartps.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
index beb151be4d32..92ec51870d1d 100644
--- 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);
-- 
2.35.1.1320.gc452695387.dirty