[PATCH] serial: 8250: force synchronous probe for the ISA and PNP drivers

[Rahul Bukte]

On x86_64 defconfig, booting with driver_async_probe=serial hangs in early
init. The 8250 PNP driver is put onto the async probe pool.
serial8250_register_8250_port() runs in a kworker concurrently with the
ISA registration done from the serial8250_init() initcall resulting in a
deadlock or NULL dereference.

  - Deadlock: serial_core_register_port() holds port_mutex across
    serial_core_add_one_port()
        uart_configure_port()
            autoconfig_irq()
                probe_irq_on()
                    async_synchronize_full(),
    which waits for the async probe pool to drain.

    The async PNP worker reaches the "port already in use" check and
    tries to unregister it.
    serial8250_register_8250_port()
        uart_remove_one_port()
            serial_core_unregister_port()
    This blocks on port_mutex.
    The init thread waits for the worker and the worker waits for the
    init thread.

  - NULL deref: when the worker instead observes a slot whose port.dev
    is set but whose port_dev has not yet been populated, it hits the
    null pointer on the call to serial_core_get_ctrl_dev() in
    serial_core_unregister_port().

Signed-off-by: Rahul Bukte <rahul.bukte@xxxxxxxx>
---
Found while experimenting with driver_async_probe=* to improve boot time.

Tested on x86_64 defconfig under QEMU + KVM with driver_async_probe=serial.

 drivers/tty/serial/8250/8250_platform.c | 1 +
 drivers/tty/serial/8250/8250_pnp.c      | 1 +
 2 files changed, 2 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_platform.c b/drivers/tty/serial/8250/8250_platform.c
index ad3a7bc31d6f..af946d12e764 100644
--- a/drivers/tty/serial/8250/8250_platform.c
+++ b/drivers/tty/serial/8250/8250_platform.c
@@ -284,6 +284,7 @@ static struct platform_driver serial8250_isa_driver = {
 	.driver		= {
 		.name	= "serial8250",
 		.acpi_match_table = acpi_platform_serial_table,
+		.probe_type = PROBE_FORCE_SYNCHRONOUS,
 	},
 };
 
diff --git a/drivers/tty/serial/8250/8250_pnp.c b/drivers/tty/serial/8250/8250_pnp.c
index 7a837fdf9df1..3f41a9d6cb27 100644
--- a/drivers/tty/serial/8250/8250_pnp.c
+++ b/drivers/tty/serial/8250/8250_pnp.c
@@ -521,6 +521,7 @@ static struct pnp_driver serial_pnp_driver = {
 	.remove		= serial_pnp_remove,
 	.driver         = {
 		.pm     = pm_sleep_ptr(&serial_pnp_pm_ops),
+		.probe_type = PROBE_FORCE_SYNCHRONOUS,
 	},
 	.id_table	= pnp_dev_table,
 };

base-commit: eb3f4b7426cfd2b79d65b7d37155480b32259a11
-- 
2.43.0