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 [ resolved merge conflicts ] Signed-off-by: Sean Anderson sean.anderson@linux.dev ---
drivers/tty/serial/xilinx_uartps.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-)
diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c index 2e5e86a00a77..7f83d2780017 100644 --- a/drivers/tty/serial/xilinx_uartps.c +++ b/drivers/tty/serial/xilinx_uartps.c @@ -268,7 +268,7 @@ 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) { @@ -369,7 +369,7 @@ 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);
- spin_unlock(&port->lock); + uart_unlock_and_check_sysrq(port); return IRQ_HANDLED; }
@@ -1229,10 +1229,8 @@ 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) - locked = spin_trylock_irqsave(&port->lock, flags); + if (oops_in_progress) + locked = uart_port_trylock_irqsave(port, &flags); else spin_lock_irqsave(&port->lock, flags);