From 6eb210ce93c5f3022c434a3d38037477ef5ec12f Mon Sep 17 00:00:00 2001 From: Thomas Gleixner <tglx@linutronix.de> Date: Sat, 16 Sep 2023 21:26:00 +0206 Subject: [PATCH 101/204] 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: John Ogness <john.ogness@linutronix.de> Signed-off-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de> Reviewed-by: Petr Mladek <pmladek@suse.com> Signed-off-by: Petr Mladek <pmladek@suse.com> Link: https://lore.kernel.org/r/20230916192007.608398-2-john.ogness@linutronix.de Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- include/linux/console.h | 31 ++++++++++++++++++++ kernel/printk/Makefile | 2 - kernel/printk/internal.h | 8 +++++ kernel/printk/nbcon.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++ kernel/printk/printk.c | 13 ++++++-- 5 files changed, 120 insertions(+), 4 deletions(-) create mode 100644 kernel/printk/nbcon.c Index: linux-6.6.58-rt45/include/linux/console.h =================================================================== @ linux-6.6.58-rt45/include/linux/console.h:159 @ 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), @ linux-6.6.58-rt45/include/linux/console.h:171 @ 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 * @write: Write callback to output messages (Optional) @ linux-6.6.58-rt45/include/linux/console.h:216 @ 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]; @ linux-6.6.58-rt45/include/linux/console.h:237 @ struct console { unsigned long dropped; void *data; struct hlist_node node; + + /* nbcon console specific members */ + atomic_t __private nbcon_state; }; #ifdef CONFIG_LOCKDEP Index: linux-6.6.58-rt45/kernel/printk/Makefile =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/Makefile +++ linux-6.6.58-rt45/kernel/printk/Makefile @ linux-6.6.58-rt45/include/linux/console.h:3 @ # 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 Index: linux-6.6.58-rt45/kernel/printk/internal.h =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/internal.h +++ linux-6.6.58-rt45/kernel/printk/internal.h @ linux-6.6.58-rt45/include/linux/console.h:6 @ * 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); @ linux-6.6.58-rt45/include/linux/console.h:65 @ 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 @ linux-6.6.58-rt45/include/linux/console.h:84 @ u16 printk_parse_prefix(const char *text #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 */ /** Index: linux-6.6.58-rt45/kernel/printk/nbcon.c =================================================================== --- /dev/null +++ linux-6.6.58-rt45/kernel/printk/nbcon.c @ linux-6.6.58-rt45/include/linux/console.h:4 @ +// 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); +} Index: linux-6.6.58-rt45/kernel/printk/printk.c =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/printk.c +++ linux-6.6.58-rt45/kernel/printk/printk.c @ linux-6.6.58-rt45/include/linux/console.h:3378 @ static void try_enable_default_console(s newcon->flags |= CON_CONSDEV; } -#define con_printk(lvl, con, fmt, ...) \ - printk(lvl pr_fmt("%sconsole [%s%d] " fmt), \ - (con->flags & CON_BOOT) ? "boot" : "", \ +#define con_printk(lvl, con, 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__) static void console_init_seq(struct console *newcon, bool bootcon_registered) @ linux-6.6.58-rt45/include/linux/console.h:3541 @ void register_console(struct console *ne 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. @ linux-6.6.58-rt45/include/linux/console.h:3635 @ static int unregister_console_locked(str */ synchronize_srcu(&console_srcu); + if (console->flags & CON_NBCON) + nbcon_cleanup(console); + console_sysfs_notify(); if (console->exit)