Commit 6b93bb41 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Petr Mladek
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printk: Add non-BKL (nbcon) console basic infrastructure 



The current console/printk subsystem is protected by a Big Kernel Lock,
(aka console_lock) which has ill defined semantics and is more or less
stateless. This puts severe limitations on the console subsystem and
makes forced takeover and output in emergency and panic situations a
fragile endeavour that is based on try and pray.

The goal of non-BKL (nbcon) consoles is to break out of the console lock
jail and to provide a new infrastructure that avoids the pitfalls and
also allows console drivers to be gradually converted over.

The proposed infrastructure aims for the following properties:

  - Per console locking instead of global locking
  - Per console state that allows to make informed decisions
  - Stateful handover and takeover

As a first step, state is added to struct console. The per console state
is an atomic_t using a 32bit bit field.

Reserve state bits, which will be populated later in the series. Wire
it up into the console register/unregister functionality.

It was decided to use a bitfield because using a plain u32 with
mask/shift operations resulted in uncomprehensible code.

Co-developed-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarThomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230916192007.608398-2-john.ogness@linutronix.de
parent 01a46efc

include/linux/console.h

+31 −0
Original line number Diff line number Diff line
@@ -156,6 +156,8 @@ static inline int con_debug_leave(void) 
 *			/dev/kmesg which requires a larger output buffer.

 * @CON_SUSPENDED:	Indicates if a console is suspended. If true, the

 *			printing callbacks must not be called.

 * @CON_NBCON:		Console can operate outside of the legacy style console_lock

 *			constraints.

 */

enum cons_flags {

	CON_PRINTBUFFER		= BIT(0),
@@ -166,8 +168,32 @@ enum cons_flags { 
	CON_BRL			= BIT(5),

	CON_EXTENDED		= BIT(6),

	CON_SUSPENDED		= BIT(7),

	CON_NBCON		= BIT(8),

};



/**

 * struct nbcon_state - console state for nbcon consoles

 * @atom:	Compound of the state fields for atomic operations

 *

 * To be used for reading and preparing of the value stored in the nbcon

 * state variable @console::nbcon_state.

 */

struct nbcon_state {

	union {

		unsigned int	atom;

		struct {

		};

	};

};



/*

 * The nbcon_state struct is used to easily create and interpret values that

 * are stored in the @console::nbcon_state variable. Ensure this struct stays

 * within the size boundaries of the atomic variable's underlying type in

 * order to avoid any accidental truncation.

 */

static_assert(sizeof(struct nbcon_state) <= sizeof(int));



/**

 * struct console - The console descriptor structure

 * @name:		The name of the console driver
@@ -187,6 +213,8 @@ enum cons_flags { 
 * @dropped:		Number of unreported dropped ringbuffer records

 * @data:		Driver private data

 * @node:		hlist node for the console list

 *

 * @nbcon_state:	State for nbcon consoles

 */

struct console {

	char			name[16];
@@ -206,6 +234,9 @@ struct console { 
	unsigned long		dropped;

	void			*data;

	struct hlist_node	node;



	/* nbcon console specific members */

	atomic_t		__private nbcon_state;

};



#ifdef CONFIG_LOCKDEP

kernel/printk/Makefile

+1 −1
Original line number Diff line number Diff line 
# SPDX-License-Identifier: GPL-2.0-only

obj-y	= printk.o

obj-$(CONFIG_PRINTK)	+= printk_safe.o

obj-$(CONFIG_PRINTK)	+= printk_safe.o nbcon.o

obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o

obj-$(CONFIG_PRINTK_INDEX)	+= index.o

kernel/printk/internal.h

+8 −0
Original line number Diff line number Diff line
@@ -3,6 +3,7 @@ 
 * internal.h - printk internal definitions

 */

#include <linux/percpu.h>

#include <linux/console.h>



#if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)

void __init printk_sysctl_init(void);
@@ -61,6 +62,10 @@ void defer_console_output(void); 


u16 printk_parse_prefix(const char *text, int *level,

			enum printk_info_flags *flags);



void nbcon_init(struct console *con);

void nbcon_cleanup(struct console *con);



#else



#define PRINTK_PREFIX_MAX	0
@@ -76,6 +81,9 @@ u16 printk_parse_prefix(const char *text, int *level, 
#define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)



static inline bool printk_percpu_data_ready(void) { return false; }

static inline void nbcon_init(struct console *con) { }

static inline void nbcon_cleanup(struct console *con) { }



#endif /* CONFIG_PRINTK */



/**

kernel/printk/nbcon.c

0 → 100644
+70 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0-only

// Copyright (C) 2022 Linutronix GmbH, John Ogness

// Copyright (C) 2022 Intel, Thomas Gleixner



#include <linux/kernel.h>

#include <linux/console.h>

#include "internal.h"

/*

 * Printk console printing implementation for consoles which does not depend

 * on the legacy style console_lock mechanism.

 */



/**

 * nbcon_state_set - Helper function to set the console state

 * @con:	Console to update

 * @new:	The new state to write

 *

 * Only to be used when the console is not yet or no longer visible in the

 * system. Otherwise use nbcon_state_try_cmpxchg().

 */

static inline void nbcon_state_set(struct console *con, struct nbcon_state *new)

{

	atomic_set(&ACCESS_PRIVATE(con, nbcon_state), new->atom);

}



/**

 * nbcon_state_read - Helper function to read the console state

 * @con:	Console to read

 * @state:	The state to store the result

 */

static inline void nbcon_state_read(struct console *con, struct nbcon_state *state)

{

	state->atom = atomic_read(&ACCESS_PRIVATE(con, nbcon_state));

}



/**

 * nbcon_state_try_cmpxchg() - Helper function for atomic_try_cmpxchg() on console state

 * @con:	Console to update

 * @cur:	Old/expected state

 * @new:	New state

 *

 * Return: True on success. False on fail and @cur is updated.

 */

static inline bool nbcon_state_try_cmpxchg(struct console *con, struct nbcon_state *cur,

					   struct nbcon_state *new)

{

	return atomic_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_state), &cur->atom, new->atom);

}



/**

 * nbcon_init - Initialize the nbcon console specific data

 * @con:	Console to initialize

 */

void nbcon_init(struct console *con)

{

	struct nbcon_state state = { };



	nbcon_state_set(con, &state);

}



/**

 * nbcon_cleanup - Cleanup the nbcon console specific data

 * @con:	Console to cleanup

 */

void nbcon_cleanup(struct console *con)

{

	struct nbcon_state state = { };



	nbcon_state_set(con, &state);

}

kernel/printk/printk.c

+10 −3
Original line number Diff line number Diff line
@@ -3327,7 +3327,8 @@ static void try_enable_default_console(struct console *newcon) 
}



#define con_printk(lvl, con, fmt, ...)				\

	printk(lvl pr_fmt("%sconsole [%s%d] " fmt),	\

	printk(lvl pr_fmt("%s%sconsole [%s%d] " fmt),		\

	       (con->flags & CON_NBCON) ? "" : "legacy ",	\

	       (con->flags & CON_BOOT) ? "boot" : "",		\

	       con->name, con->index, ##__VA_ARGS__)
@@ -3488,6 +3489,9 @@ void register_console(struct console *newcon) 
	newcon->dropped = 0;

	console_init_seq(newcon, bootcon_registered);



	if (newcon->flags & CON_NBCON)

		nbcon_init(newcon);



	/*

	 * Put this console in the list - keep the

	 * preferred driver at the head of the list.
@@ -3579,6 +3583,9 @@ static int unregister_console_locked(struct console *console) 
	 */

	synchronize_srcu(&console_srcu);



	if (console->flags & CON_NBCON)

		nbcon_cleanup(console);



	console_sysfs_notify();



	if (console->exit)