serial: 8250: dispatch SysRq character in serial8250_handle_irq()

serial8250_handle_irq() captures a SysRq character into port->sysrq_ch
inside serial8250_handle_irq_locked() via uart_prepare_sysrq_char()
(reached from serial8250_read_char()). Dispatch of that captured
character to handle_sysrq() is expected to happen at port-unlock time,
through uart_unlock_and_check_sysrq[_irqrestore]().

After commit 8324a54f604d ("serial: 8250: Add
serial8250_handle_irq_locked()") the function was reduced to a wrapper
that takes the port lock via guard(uart_port_lock_irqsave) whose
destructor is plain uart_port_unlock_irqrestore(). The sysrq-aware
unlock helper is no longer called, so port->sysrq_ch is captured but
never dispatched: BREAK + SysRq key is consumed silently.

This was the very condition Johan Hovold's 853a9ae29e978 ("serial:
8250: fix handle_irq locking", 2021) introduced
uart_unlock_and_check_sysrq_irqrestore() to address.

Switch to the new guard(uart_port_lock_check_sysrq_irqsave), whose
destructor is the sysrq-aware unlock helper, restoring the pre-split
behaviour. Update the Context: comment on serial8250_handle_irq_locked()
so future HW-specific 8250 wrappers know to use the same guard or the
explicit sysrq-aware unlock.

Verified on RTL8196E with CONFIG_MAGIC_SYSRQ_SERIAL=y: BREAK + 'h' on
the console UART produces the SysRq help dump in dmesg and the brk
counter in /proc/tty/driver/serial increments correctly.

Fixes: 8324a54f604d ("serial: 8250: Add serial8250_handle_irq_locked()")
Cc: stable@vger.kernel.org
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jacques Nilo <jnilo@free.fr>
Link: https://patch.msgid.link/52692ae6c3501f7940347cef364ad7fcacaab7e5.1778675349.git.jnilo@free.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index af78cc02f38e719573becd0aea226f7790555a3e..c66ba714caa5f7676702666815dd04ef23379164 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1784,7 +1784,10 @@ static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
 }
 
 /*
- * Context: port's lock must be held by the caller.
+ * Context: port's lock must be held by the caller. The caller must
+ * release it via guard(uart_port_lock_check_sysrq_irqsave) or
+ * uart_unlock_and_check_sysrq_irqrestore(), which captures SysRq
+ * character on unlock.
  */
 void serial8250_handle_irq_locked(struct uart_port *port, unsigned int iir)
 {
@@ -1837,7 +1840,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
        if (iir & UART_IIR_NO_INT)
                return 0;
 
-       guard(uart_port_lock_irqsave)(port);
+       guard(uart_port_lock_check_sysrq_irqsave)(port);
        serial8250_handle_irq_locked(port, iir);
 
        return 1;