serial: core: fix console port-lock regression

Quoted 6 lines · Johan Hovold

Commit f743061a85f5 ("serial: core: Initialise spin lock before use in uart_configure_port()") tried to work around a breakage introduced by commit a3cb39d258ef ("serial: core: Allow detach and attach serial device for console") by adding a second initialisation of the port lock when registering the port.

Quoted 3 lines

As reported by the build robots [1], this doesn't really solve the regression introduced by the console-detach changes and also adds a second redundant initialisation of the lock for normal ports.

I thought, though doubtfully, it was a regression made by 679193b7baf8 ("serial: 8250: Let serial core initialise spin lock") and then I completely forgot about [1].

Quoted 3 lines

Start cleaning up this mess by removing the redundant initialisation and making sure that the port lock is again initialised once-only for ports that aren't already in use as a console.

Thanks for looking into this!

I agree this is better place for lock initialization.

Reviewed-by Andy Shevchenko <[email protected]>

Quoted 16 lines · Johan Hovold

Fix the port-lock initialisation regression introduced by commit a3cb39d258ef ("serial: core: Allow detach and attach serial device for console") by making sure that the lock is again initialised during console setup.

The console may be registered before the serial controller has been probed in which case the port lock needs to be initialised during console setup by a call to uart_set_options(). The console-detach changes introduced a regression in several drivers by effectively removing that initialisation by not initialising the lock when the port is used as a console (which is always the case during console setup).

Add back the early lock initialisation and instead use a new console-reinit flag to handle the case where a console is being re-attached through sysfs.

Quoted 2 lines

The question whether the console-detach interface should have been added in the first place is left for another discussion.

It was discussed in [1]. TL;DR: OMAP would like to keep runtime PM available for UART while at the same time we disable it for kernel consoles in bedb404e91bb.

[1]: https://lists.openwall.net/linux-kernel/2018/09/29/65

Quoted 4 lines

Note that the console-enabled check in uart_set_options() is not redundant because of kgdboc, which can end up reinitialising an already enabled console (see commit 42b6a1baa3ec ("serial_core: Don't re-initialize a previously initialized spinlock.")).

Reviewed-by Andy Shevchenko <[email protected]>

Thank you!

One question below, though.

Quoted 30 lines

Fixes a3cb39d258ef ("serial: core: Allow detach and attach serial device for console")

Cc stable <[email protected]> # 5.7

Cc Andy Shevchenko <[email protected]>

Signed-off-by Johan Hovold <[email protected]>

---
 drivers/tty/serial/serial_core.c | 32 +++++++++++++++-----------------
 include/linux/serial_core.h      |  1 +
 2 files changed, 16 insertions(+), 17 deletions(-)

drivers/tty/serial/serial_core.c +1 -7

index 53b79e1fcbc8..124524ecfe26 100644
@@ -1916,24 +1916,12 @@ static inline bool uart_console_enabled(struct uart_port *port)
 	return uart_console(port) && (port->cons->flags & CON_ENABLED);
 }
 
-static void __uart_port_spin_lock_init(struct uart_port *port)
+static void uart_port_spin_lock_init(struct uart_port *port)
 {
 	spin_lock_init(&port->lock);
 	lockdep_set_class(&port->lock, &port_lock_key);
 }
 
-/*
- * Ensure that the serial console lock is initialised early.
- * If this port is a console, then the spinlock is already initialised.
- */
-static inline void uart_port_spin_lock_init(struct uart_port *port)
-{

drivers/tty/serial/serial_core.c static inline bool uart_console_enabled(struct uart_port *port) · line 1931

Quoted 1 line

drivers/tty/serial/serial_core.c +0 -1

-	if (uart_console(port))

I'm wondering if we may revert this line to be uart_console_enabled() and use a helper below twice.

Quoted 55 lines

drivers/tty/serial/serial_core.c +14 -9

-		return;
-
-	__uart_port_spin_lock_init(port);
-}
-
 #if defined(CONFIG_SERIAL_CORE_CONSOLE) || defined(CONFIG_CONSOLE_POLL)
 /**
  *	uart_console_write - write a console message to a serial port

@@ -2086,7 +2074,15 @@ uart_set_options(struct uart_port *port, struct console *co,
 	struct ktermios termios;
 	static struct ktermios dummy;
 
-	uart_port_spin_lock_init(port);
+	/*
+	 * Ensure that the serial-console lock is initialised early.
+	 *
+	 * Note that the console-enabled check is needed because of kgdboc,
+	 * which can end up calling uart_set_options() for an already enabled
+	 * console via tty_find_polling_driver() and uart_poll_init().
+	 */
+	if (!uart_console_enabled(port) && !port->console_reinit)
+		uart_port_spin_lock_init(port);
 
 	memset(&termios, 0, sizeof(struct ktermios));
 

@@ -2794,10 +2790,12 @@ static ssize_t console_store(struct device *dev,
 		if (oldconsole && !newconsole) {
 			ret = unregister_console(uport->cons);
 		} else if (!oldconsole && newconsole) {
-			if (uart_console(uport))
+			if (uart_console(uport)) {
+				uport->console_reinit = 1;
 				register_console(uport->cons);
-			else
+			} else {
 				ret = -ENOENT;
+			}
 		}
 	} else {
 		ret = -ENXIO;

@@ -2898,7 +2896,7 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 	 * initialised.
 	 */
 	if (!uart_console_enabled(uport))
-		__uart_port_spin_lock_init(uport);
+		uart_port_spin_lock_init(uport);
 
 	if (uport->cons && uport->dev)
 		of_console_check(uport->dev->of_node, uport->cons->name, uport->line);

include/linux/serial_core.h +0 -0

index 01fc4d9c9c54..8a99279a579b 100644
@@ -248,6 +248,7 @@ struct uart_port {
 

Quoted 8 lines

include/linux/serial_core.h +1 -0

 	unsigned char		hub6;			/* this should be in the 8250 driver */
 	unsigned char		suspended;
+	unsigned char		console_reinit;
 	const char		*name;			/* port name */
 	struct attribute_group	*attr_group;		/* port specific attributes */
 	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */

Signature · 1 line

2.26.2

Quoted 24 lines · Andy Shevchenko

Quoted 12 lines · Johan Hovold

Fix the port-lock initialisation regression introduced by commit a3cb39d258ef ("serial: core: Allow detach and attach serial device for console") by making sure that the lock is again initialised during console setup.

The console may be registered before the serial controller has been probed in which case the port lock needs to be initialised during console setup by a call to uart_set_options(). The console-detach changes introduced a regression in several drivers by effectively removing that initialisation by not initialising the lock when the port is used as a console (which is always the case during console setup).

Quoted 6 lines

Add back the early lock initialisation and instead use a new console-reinit flag to handle the case where a console is being re-attached through sysfs.

The question whether the console-detach interface should have been added in the first place is left for another discussion.

It was discussed in [1]. TL;DR: OMAP would like to keep runtime PM available for UART while at the same time we disable it for kernel consoles in bedb404e91bb.

Quoted 1 line

[1]: https://lists.openwall.net/linux-kernel/2018/09/29/65

Yeah, I remember that. My fear is just that the new interface opens up a can of worms as it removes the earlier assumption that the console would essentially never be deregistered without really fixing all those drivers, and core functions, written under that assumption. Just to mention a few issues; we have drivers enabling clocks and other resources during console setup which can now be done repeatedly,

and
several drivers whose setup callbacks are marked __init and will oops the minute you reattach the console.

And what about power management
which was the reason for wanting this on OMAP in the first place; tty core never calls shutdown() for a console port, not even when it's been detached using the new interface.

I know, the console setup is all a mess, but this still seems a little rushed to me. I'm even inclined to suggest a revert until the above and similar issues have been addressed properly rather keeping a known buggy interface.

Quoted 4 lines

Quoted 4 lines

Note that the console-enabled check in uart_set_options() is not redundant because of kgdboc, which can end up reinitialising an already enabled console (see commit 42b6a1baa3ec ("serial_core: Don't re-initialize a previously initialized spinlock.")).

+Cc: Tony, let me add Tony to the discussion.

Quoted 25 lines · Johan Hovold

Quoted 24 lines · Andy Shevchenko

Quoted 12 lines · Johan Hovold

Fix the port-lock initialisation regression introduced by commit a3cb39d258ef ("serial: core: Allow detach and attach serial device for console") by making sure that the lock is again initialised during console setup.

The console may be registered before the serial controller has been probed in which case the port lock needs to be initialised during console setup by a call to uart_set_options(). The console-detach changes introduced a regression in several drivers by effectively removing that initialisation by not initialising the lock when the port is used as a console (which is always the case during console setup).

Quoted 6 lines

Add back the early lock initialisation and instead use a new console-reinit flag to handle the case where a console is being re-attached through sysfs.

The question whether the console-detach interface should have been added in the first place is left for another discussion.

It was discussed in [1]. TL;DR: OMAP would like to keep runtime PM available for UART while at the same time we disable it for kernel consoles in bedb404e91bb.

Quoted 8 lines

Quoted 1 line

[1]: https://lists.openwall.net/linux-kernel/2018/09/29/65

Yeah, I remember that. My fear is just that the new interface opens up a can of worms as it removes the earlier assumption that the console would essentially never be deregistered without really fixing all those drivers, and core functions, written under that assumption. Just to mention a few issues; we have drivers enabling clocks and other resources during console setup which can now be done repeatedly,

The series introduced the console ->exit() callback, so it should be easy to fix.

Quoted 3 lines

and
several drivers whose setup callbacks are marked __init and will oops the minute you reattach the console.

I believe this can be fixed relatively easy. As a last resort it can be a quirk that disables console detachment for problematic consoles.

Quoted 4 lines

And what about power management
which was the reason for wanting this on OMAP in the first place; tty core never calls shutdown() for a console port, not even when it's been detached using the new interface.

That is interesting... Tony, do we have OMAP case working because of luck?

Quoted 4 lines

I know, the console setup is all a mess, but this still seems a little rushed to me. I'm even inclined to suggest a revert until the above and similar issues have been addressed properly rather keeping a known buggy interface.

You know that it will be a dead end. Any solution how to move forward?

+Cc: Tony, let me add Tony to the discussion.

Quoted 4 lines · Johan Hovold

And what about power management
which was the reason for wanting this on OMAP in the first place; tty core never calls shutdown() for a console port, not even when it's been detached using the new interface.

That is interesting... Tony, do we have OMAP case working because of luck?

Quoted 9 lines · Andy Shevchenko

+Cc: Tony, let me add Tony to the discussion.

Quoted 4 lines · Johan Hovold

And what about power management
which was the reason for wanting this on OMAP in the first place; tty core never calls shutdown() for a console port, not even when it's been detached using the new interface.

That is interesting... Tony, do we have OMAP case working because of luck?

8250_omap won't do anything unless autosuspend_timeout is configured for the uart(s). If configured, then the 8250_omap will idle when console is detached and the PM runtime usage count held by console is decremented, and the configured autosuspend_timeout expires.

The console is still kept open by getty, so I don't see why shutdown() would be called for the console port. But maybe I don't follow what you're concerned about, let me know if you want me to check something :)

Quoted 16 lines · Tony Lindgren

Quoted 9 lines · Andy Shevchenko

+Cc: Tony, let me add Tony to the discussion.

Quoted 4 lines · Johan Hovold

And what about power management
which was the reason for wanting this on OMAP in the first place; tty core never calls shutdown() for a console port, not even when it's been detached using the new interface.

That is interesting... Tony, do we have OMAP case working because of luck?

8250_omap won't do anything unless autosuspend_timeout is configured for the uart(s). If configured, then the 8250_omap will idle when console is detached and the PM runtime usage count held by console is decremented, and the configured autosuspend_timeout expires.

Quoted 3 lines

The console is still kept open by getty, so I don't see why shutdown() would be called for the console port. But maybe I don't follow what you're concerned about, let me know if you want me to check something :)

Is it possible to test configuration when you have kernel console enabled but no getty is run on it (perhaps something with ssh enabled access)?

Then kernel console should call ->shutdown on detaching, right?