From: Thomas Gleixner <[email protected]>
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 <[email protected]>
Signed-off-by: John Ogness <[email protected]>
Signed-off-by: Thomas Gleixner (Intel) <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
---
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
diff --git a/include/linux/console.h b/include/linux/console.h
index 7de11c763eb3..a2d37a7a98a8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -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
diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
index f5b388e810b9..39a2b61c7232 100644
--- a/kernel/printk/Makefile
+++ b/kernel/printk/Makefile
@@ -1,6 +1,6 @@
# 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
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 7d4979d5c3ce..2ca0ab78802c 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -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 */
/**
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
new file mode 100644
index 000000000000..10266d3e7883
--- /dev/null
+++ b/kernel/printk/nbcon.c
@@ -0,0 +1,70 @@
+// 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 that do 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);
+}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8787d3a72114..c0246093ea41 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3324,9 +3324,10 @@ static void try_enable_default_console(struct console *newcon)
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)
@@ -3486,6 +3487,9 @@ 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.
@@ -3577,6 +3581,9 @@ 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)
--
2.39.2