Merge tag 'printk-for-6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Add NMI-safe SRCU reader API. It uses atomic_inc() instead of
   this_cpu_inc() on strong load-store architectures.

 - Introduce new console_list_lock to synchronize a manipulation of the
   list of registered consoles and their flags.

   This is a first step in removing the big-kernel-lock-like behavior of
   console_lock(). This semaphore still serializes console->write()
   calbacks against:

      - each other. It primary prevents potential races between early
        and proper console drivers using the same device.

      - suspend()/resume() callbacks and init() operations in some
        drivers.

      - various other operations in the tty/vt and framebufer
        susbsystems. It is likely that console_lock() serializes even
        operations that are not directly conflicting with the
        console->write() callbacks here. This is the most complicated
        big-kernel-lock aspect of the console_lock() that will be hard
        to untangle.

 - Introduce new console_srcu lock that is used to safely iterate and
   access the registered console drivers under SRCU read lock.

   This is a prerequisite for introducing atomic console drivers and
   console kthreads. It will reduce the complexity of serialization
   against normal consoles and console_lock(). Also it should remove the
   risk of deadlock during critical situations, like Oops or panic, when
   only atomic consoles are registered.

 - Check whether the console is registered instead of enabled on many
   locations. It was a historical leftover.

 - Cleanly force a preferred console in xenfb code instead of a dirty
   hack.

 - A lot of code and comment clean ups and improvements.

* tag 'printk-for-6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux: (47 commits)
  printk: htmldocs: add missing description
  tty: serial: sh-sci: use setup() callback for early console
  printk: relieve console_lock of list synchronization duties
  tty: serial: kgdboc: use console_list_lock to trap exit
  tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()
  tty: serial: kgdboc: use console_list_lock for list traversal
  tty: serial: kgdboc: use srcu console list iterator
  proc: consoles: use console_list_lock for list iteration
  tty: tty_io: use console_list_lock for list synchronization
  printk, xen: fbfront: create/use safe function for forcing preferred
  netconsole: avoid CON_ENABLED misuse to track registration
  usb: early: xhci-dbc: use console_is_registered()
  tty: serial: xilinx_uartps: use console_is_registered()
  tty: serial: samsung_tty: use console_is_registered()
  tty: serial: pic32_uart: use console_is_registered()
  tty: serial: earlycon: use console_is_registered()
  tty: hvc: use console_is_registered()
  efi: earlycon: use console_is_registered()
  tty: nfcon: use console_is_registered()
  serial_core: replace uart_console_enabled() with uart_console_registered()
  ...

4 files changed, 133 insertions, 19 deletions

diff --git a/include/linux/console.h b/include/linux/console.h
index 8c1686e2c233..9cea254b34b8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -15,6 +15,7 @@
 #define _LINUX_CONSOLE_H_ 1
 
 #include <linux/atomic.h>
+#include <linux/rculist.h>
 #include <linux/types.h>
 
 struct vc_data;
@@ -154,14 +155,132 @@ struct console {
 	u64	seq;
 	unsigned long dropped;
 	void	*data;
-	struct	 console *next;
+	struct hlist_node node;
 };
 
+#ifdef CONFIG_LOCKDEP
+extern void lockdep_assert_console_list_lock_held(void);
+#else
+static inline void lockdep_assert_console_list_lock_held(void)
+{
+}
+#endif
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+extern bool console_srcu_read_lock_is_held(void);
+#else
+static inline bool console_srcu_read_lock_is_held(void)
+{
+	return 1;
+}
+#endif
+
+extern int console_srcu_read_lock(void);
+extern void console_srcu_read_unlock(int cookie);
+
+extern void console_list_lock(void) __acquires(console_mutex);
+extern void console_list_unlock(void) __releases(console_mutex);
+
+extern struct hlist_head console_list;
+
+/**
+ * console_srcu_read_flags - Locklessly read the console flags
+ * @con:	struct console pointer of console to read flags from
+ *
+ * This function provides the necessary READ_ONCE() and data_race()
+ * notation for locklessly reading the console flags. The READ_ONCE()
+ * in this function matches the WRITE_ONCE() when @flags are modified
+ * for registered consoles with console_srcu_write_flags().
+ *
+ * Only use this function to read console flags when locklessly
+ * iterating the console list via srcu.
+ *
+ * Context: Any context.
+ */
+static inline short console_srcu_read_flags(const struct console *con)
+{
+	WARN_ON_ONCE(!console_srcu_read_lock_is_held());
+
+	/*
+	 * Locklessly reading console->flags provides a consistent
+	 * read value because there is at most one CPU modifying
+	 * console->flags and that CPU is using only read-modify-write
+	 * operations to do so.
+	 */
+	return data_race(READ_ONCE(con->flags));
+}
+
+/**
+ * console_srcu_write_flags - Write flags for a registered console
+ * @con:	struct console pointer of console to write flags to
+ * @flags:	new flags value to write
+ *
+ * Only use this function to write flags for registered consoles. It
+ * requires holding the console_list_lock.
+ *
+ * Context: Any context.
+ */
+static inline void console_srcu_write_flags(struct console *con, short flags)
+{
+	lockdep_assert_console_list_lock_held();
+
+	/* This matches the READ_ONCE() in console_srcu_read_flags(). */
+	WRITE_ONCE(con->flags, flags);
+}
+
+/* Variant of console_is_registered() when the console_list_lock is held. */
+static inline bool console_is_registered_locked(const struct console *con)
+{
+	lockdep_assert_console_list_lock_held();
+	return !hlist_unhashed(&con->node);
+}
+
 /*
- * for_each_console() allows you to iterate on each console
+ * console_is_registered - Check if the console is registered
+ * @con:	struct console pointer of console to check
+ *
+ * Context: Process context. May sleep while acquiring console list lock.
+ * Return: true if the console is in the console list, otherwise false.
+ *
+ * If false is returned for a console that was previously registered, it
+ * can be assumed that the console's unregistration is fully completed,
+ * including the exit() callback after console list removal.
+ */
+static inline bool console_is_registered(const struct console *con)
+{
+	bool ret;
+
+	console_list_lock();
+	ret = console_is_registered_locked(con);
+	console_list_unlock();
+	return ret;
+}
+
+/**
+ * for_each_console_srcu() - Iterator over registered consoles
+ * @con:	struct console pointer used as loop cursor
+ *
+ * Although SRCU guarantees the console list will be consistent, the
+ * struct console fields may be updated by other CPUs while iterating.
+ *
+ * Requires console_srcu_read_lock to be held. Can be invoked from
+ * any context.
+ */
+#define for_each_console_srcu(con)					\
+	hlist_for_each_entry_srcu(con, &console_list, node,		\
+				  console_srcu_read_lock_is_held())
+
+/**
+ * for_each_console() - Iterator over registered consoles
+ * @con:	struct console pointer used as loop cursor
+ *
+ * The console list and the console->flags are immutable while iterating.
+ *
+ * Requires console_list_lock to be held.
  */
-#define for_each_console(con) \
-	for (con = console_drivers; con != NULL; con = con->next)
+#define for_each_console(con)						\
+	lockdep_assert_console_list_lock_held();			\
+	hlist_for_each_entry(con, &console_list, node)
 
 extern int console_set_on_cmdline;
 extern struct console *early_console;
@@ -172,9 +291,9 @@ enum con_flush_mode {
 };
 
 extern int add_preferred_console(char *name, int idx, char *options);
+extern void console_force_preferred_locked(struct console *con);
 extern void register_console(struct console *);
 extern int unregister_console(struct console *);
-extern struct console *console_drivers;
 extern void console_lock(void);
 extern int console_trylock(void);
 extern void console_unlock(void);
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 5c271bf3a1e7..68f9070aa111 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -152,11 +152,7 @@ static inline bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 	return false;
 }
 static inline void rcu_preempt_deferred_qs(struct task_struct *t) { }
-#ifdef CONFIG_SRCU
 void rcu_scheduler_starting(void);
-#else /* #ifndef CONFIG_SRCU */
-static inline void rcu_scheduler_starting(void) { }
-#endif /* #else #ifndef CONFIG_SRCU */
 static inline void rcu_end_inkernel_boot(void) { }
 static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
 static inline bool rcu_is_watching(void) { return true; }
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index d657f2a42a7b..91871464b99d 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -743,9 +743,15 @@ static const bool earlycon_acpi_spcr_enable EARLYCON_USED_OR_UNUSED;
 static inline int setup_earlycon(char *buf) { return 0; }
 #endif
 
-static inline bool uart_console_enabled(struct uart_port *port)
+/* Variant of uart_console_registered() when the console_list_lock is held. */
+static inline bool uart_console_registered_locked(struct uart_port *port)
 {
-	return uart_console(port) && (port->cons->flags & CON_ENABLED);
+	return uart_console(port) && console_is_registered_locked(port->cons);
+}
+
+static inline bool uart_console_registered(struct uart_port *port)
+{
+	return uart_console(port) && console_is_registered(port->cons);
 }
 
 struct uart_port *uart_get_console(struct uart_port *ports, int nr,
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index f0814ffca34b..9b9d0bbf1d3c 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -47,11 +47,8 @@ int init_srcu_struct(struct srcu_struct *ssp);
 #include <linux/srcutiny.h>
 #elif defined(CONFIG_TREE_SRCU)
 #include <linux/srcutree.h>
-#elif defined(CONFIG_SRCU)
-#error "Unknown SRCU implementation specified to kernel configuration"
 #else
-/* Dummy definition for things like notifiers.  Actual use gets link error. */
-struct srcu_struct { };
+#error "Unknown SRCU implementation specified to kernel configuration"
 #endif
 
 void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
@@ -78,11 +75,7 @@ static inline void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 }
 #endif /* CONFIG_NEED_SRCU_NMI_SAFE */
 
-#ifdef CONFIG_SRCU
 void srcu_init(void);
-#else /* #ifdef CONFIG_SRCU */
-static inline void srcu_init(void) { }
-#endif /* #else #ifdef CONFIG_SRCU */
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC