[RFC PATCH] serial: core: Use pm_runtime_get_sync() in uart_start()

[Xuewen Yan]

The commit 84a9582fd203("serial: core: Start managing serial controllers to enable runtime PM")
use the pm_runtime_get() after uart_port_lock() which would close the irq and disable preement.
At this time,  pm_runtime_get may cause the following two problems:

(1) deadlock in try_to_wake_up:

uart_write()
uart_port_lock() <<< get lock
__uart_start
__pm_runtime_resume
rpm_resume
queue_work_on
try_to_wake_up
_printk
uart_console_write
...
uart_port_lock() <<< wait forever

(2) scheduling while atomic:
uart_write()
uart_port_lock() <<< get lock
__uart_start
__pm_runtime_resume
rpm_resume
chedule() << sleep

So let us use pm_runtime_get_sync() to prevent these.

Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
---
 drivers/tty/serial/serial_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index f1348a509552..902f7ed35f4d 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -145,7 +145,7 @@ static void __uart_start(struct uart_state *state)
 	port_dev = port->port_dev;
 
 	/* Increment the runtime PM usage count for the active check below */
-	err = pm_runtime_get(&port_dev->dev);
+	err = pm_runtime_get_sync(&port_dev->dev);
 	if (err < 0 && err != -EINPROGRESS) {
 		pm_runtime_put_noidle(&port_dev->dev);
 		return;
@@ -159,7 +159,7 @@ static void __uart_start(struct uart_state *state)
 	if (!pm_runtime_enabled(port->dev) || pm_runtime_active(port->dev))
 		port->ops->start_tx(port);
 	pm_runtime_mark_last_busy(&port_dev->dev);
-	pm_runtime_put_autosuspend(&port_dev->dev);
+	pm_runtime_put_sync_autosuspend(&port_dev->dev);
 }
 
 static void uart_start(struct tty_struct *tty)
-- 
2.25.1