Merge tag 'printk-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:
 "This is the "last" part of the support for the new nbcon consoles.
  Where "nbcon" stays for "No Big console lock CONsoles" aka not under
  the console_lock.

  New callbacks are added to struct console:

   - write_thread() for flushing nbcon consoles in task context.

   - write_atomic() for flushing nbcon consoles in atomic context,
     including NMI.

   - con->device_lock() and device_unlock() for taking the driver
     specific lock, for example, port->lock.

  New printk-specific kthreads are created:

   - per-console kthreads which get responsible for flushing normal
     priority messages on nbcon consoles.

   - thread which gets responsible for flushing normal priority messages
     on all consoles when CONFIG_RT enabled.

  The new callbacks are called under a special per-console lock which
  has already been added back in v6.7. It allows to distinguish three
  severities: normal, emergency, and panic. A context with a higher
  priority could take over the ownership when it is safe even in the
  middle of handling a record. The panic context could do it even when
  it is not safe. But it is allowed only for the final desperate flush
  before entering the infinite loop.

  The new lock helps to flush the messages directly in emergency and
  panic contexts. But it is not enough in all situations:

   - console_lock() is still need for synchronization against boot
     consoles.

   - con->device_lock() is need for synchronization against other
     operations on the same HW, e.g. serial port speed setting,
     non-printk related read/write.

  The dependency on con->device_lock() is mutual. Any code taking the
  driver specific lock has to acquire the related nbcon console context
  as well. For example, see the new uart_port_lock() API. It provides
  the necessary synchronization against emergency and panic contexts
  where the messages are flushed only under the new per-console lock.

  Maybe surprisingly, a quite tricky part is the decision how to flush
  the consoles in various situations. It has to take into account:

   - message priority:    normal, emergency, panic

   - scheduling context:  task, atomic, deferred_legacy

   - registered consoles: boot, legacy, nbcon

   - threads are running: early boot, suspend, shutdown, panic

   - caller:              printk(), pr_flush(), printk_flush_in_panic(),
                          console_unlock(), console_start(), ...

  The primary decision is made in printk_get_console_flush_type(). It
  creates a hint what the caller should do:

   - flush nbcon consoles directly or via the kthread

   - call the legacy loop (console_unlock()) directly or via irq_work

  The existing behavior is preserved for the legacy consoles. The only
  exception is that they are not longer flushed directly from printk()
  in panic() before CPUs are stopped. But this blocking happens only
  when at least one nbcon console is registered. The motivation is to
  increase a chance to produce the crash dump. They legacy consoles
  might create a deadlock in compare with nbcon consoles. The nbcon
  console should allow to see the messages even when the crash dump
  fails.

  There are three possible ways how nbcon consoles are flushed:

   - The per-nbcon-console kthread is responsible for flushing messages
     added with the normal priority. This is the default mode.

   - The legacy loop, aka console_unlock(), is used when there is still
     a boot console registered. There is no easy way how to match an
     early console driver with a nbcon console driver. And the
     console_lock() provides the only reliable serialization at the
     moment.

     The legacy loop uses either con->write_atomic() or
     con->write_thread() callbacks depending on whether it is allowed to
     schedule. The atomic variant has to be used from printk().

   - In other situations, the messages are flushed directly using
     write_atomic() which can be called in any context, including NMI.
     It is primary needed during early boot or shutdown, in emergency
     situations, and panic.

  The emergency priority is used by a code called within
  nbcon_cpu_emergency_enter()/exit(). At the moment, it is used in four
  situations: WARN(), Oops, lockdep, and RCU stall reports.

  Finally, there is no nbcon console at the moment. It means that the
  changes should _not_ modify the existing behavior. The only exception
  is CONFIG_RT which would force offloading the legacy loop, for normal
  priority context, into the dedicated kthread"

* tag 'printk-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux: (54 commits)
  printk: Avoid false positive lockdep report for legacy printing
  printk: nbcon: Assign nice -20 for printing threads
  printk: Implement legacy printer kthread for PREEMPT_RT
  tty: sysfs: Add nbcon support for 'active'
  proc: Add nbcon support for /proc/consoles
  proc: consoles: Add notation to c_start/c_stop
  printk: nbcon: Show replay message on takeover
  printk: Provide helper for message prepending
  printk: nbcon: Rely on kthreads for normal operation
  printk: nbcon: Use thread callback if in task context for legacy
  printk: nbcon: Relocate nbcon_atomic_emit_one()
  printk: nbcon: Introduce printer kthreads
  printk: nbcon: Init @nbcon_seq to highest possible
  printk: nbcon: Add context to usable() and emit()
  printk: Flush console on unregister_console()
  printk: Fail pr_flush() if before SYSTEM_SCHEDULING
  printk: nbcon: Add function for printers to reacquire ownership
  printk: nbcon: Use raw_cpu_ptr() instead of open coding
  printk: Use the BITS_PER_LONG macro
  lockdep: Mark emergency sections in lockdep splats
  ...

1 files changed, 602 insertions, 110 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6ff8d47e145f..71e4fe6f9b85 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -34,6 +34,7 @@
 #include <linux/security.h>
 #include <linux/memblock.h>
 #include <linux/syscalls.h>
+#include <linux/syscore_ops.h>
 #include <linux/vmcore_info.h>
 #include <linux/ratelimit.h>
 #include <linux/kmsg_dump.h>
@@ -282,6 +283,7 @@ EXPORT_SYMBOL(console_list_unlock);
  * Return: A cookie to pass to console_srcu_read_unlock().
  */
 int console_srcu_read_lock(void)
+	__acquires(&console_srcu)
 {
 	return srcu_read_lock_nmisafe(&console_srcu);
 }
@@ -295,6 +297,7 @@ EXPORT_SYMBOL(console_srcu_read_lock);
  * Counterpart to console_srcu_read_lock()
  */
 void console_srcu_read_unlock(int cookie)
+	__releases(&console_srcu)
 {
 	srcu_read_unlock_nmisafe(&console_srcu, cookie);
 }
@@ -461,14 +464,43 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 /* syslog_lock protects syslog_* variables and write access to clear_seq. */
 static DEFINE_MUTEX(syslog_lock);
 
+/*
+ * Specifies if a legacy console is registered. If legacy consoles are
+ * present, it is necessary to perform the console lock/unlock dance
+ * whenever console flushing should occur.
+ */
+bool have_legacy_console;
+
+/*
+ * Specifies if an nbcon console is registered. If nbcon consoles are present,
+ * synchronous printing of legacy consoles will not occur during panic until
+ * the backtrace has been stored to the ringbuffer.
+ */
+bool have_nbcon_console;
+
+/*
+ * Specifies if a boot console is registered. If boot consoles are present,
+ * nbcon consoles cannot print simultaneously and must be synchronized by
+ * the console lock. This is because boot consoles and nbcon consoles may
+ * have mapped the same hardware.
+ */
+bool have_boot_console;
+
+/* See printk_legacy_allow_panic_sync() for details. */
+bool legacy_allow_panic_sync;
+
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
+static DECLARE_WAIT_QUEUE_HEAD(legacy_wait);
 /* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
 static u64 syslog_seq;
 static size_t syslog_partial;
 static bool syslog_time;
 
+/* True when _all_ printer threads are available for printing. */
+bool printk_kthreads_running;
+
 struct latched_seq {
 	seqcount_latch_t	latch;
 	u64			val[2];
@@ -1850,7 +1882,7 @@ static bool console_waiter;
  * there may be a waiter spinning (like a spinlock). Also it must be
  * ready to hand over the lock at the end of the section.
  */
-static void console_lock_spinning_enable(void)
+void console_lock_spinning_enable(void)
 {
 	/*
 	 * Do not use spinning in panic(). The panic CPU wants to keep the lock.
@@ -1889,7 +1921,7 @@ lockdep:
  *
  * Return: 1 if the lock rights were passed, 0 otherwise.
  */
-static int console_lock_spinning_disable_and_check(int cookie)
+int console_lock_spinning_disable_and_check(int cookie)
 {
 	int waiter;
 
@@ -2300,12 +2332,30 @@ out:
 	return ret;
 }
 
+/*
+ * This acts as a one-way switch to allow legacy consoles to print from
+ * the printk() caller context on a panic CPU. It also attempts to flush
+ * the legacy consoles in this context.
+ */
+void printk_legacy_allow_panic_sync(void)
+{
+	struct console_flush_type ft;
+
+	legacy_allow_panic_sync = true;
+
+	printk_get_console_flush_type(&ft);
+	if (ft.legacy_direct) {
+		if (console_trylock())
+			console_unlock();
+	}
+}
+
 asmlinkage int vprintk_emit(int facility, int level,
 			    const struct dev_printk_info *dev_info,
 			    const char *fmt, va_list args)
 {
+	struct console_flush_type ft;
 	int printed_len;
-	bool in_sched = false;
 
 	/* Suppress unimportant messages after panic happens */
 	if (unlikely(suppress_printk))
@@ -2319,17 +2369,26 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (other_cpu_in_panic() && !panic_triggering_all_cpu_backtrace)
 		return 0;
 
+	printk_get_console_flush_type(&ft);
+
+	/* If called from the scheduler, we can not call up(). */
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
-		in_sched = true;
+		ft.legacy_offload |= ft.legacy_direct;
+		ft.legacy_direct = false;
 	}
 
 	printk_delay(level);
 
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
-	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched) {
+	if (ft.nbcon_atomic)
+		nbcon_atomic_flush_pending();
+
+	if (ft.nbcon_offload)
+		nbcon_kthreads_wake();
+
+	if (ft.legacy_direct) {
 		/*
 		 * The caller may be holding system-critical or
 		 * timing-sensitive locks. Disable preemption during
@@ -2349,7 +2408,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	if (in_sched)
+	if (ft.legacy_offload)
 		defer_console_output();
 	else
 		wake_up_klogd();
@@ -2678,6 +2737,7 @@ void suspend_console(void)
 
 void resume_console(void)
 {
+	struct console_flush_type ft;
 	struct console *con;
 
 	if (!console_suspend_enabled)
@@ -2695,6 +2755,12 @@ void resume_console(void)
 	 */
 	synchronize_srcu(&console_srcu);
 
+	printk_get_console_flush_type(&ft);
+	if (ft.nbcon_offload)
+		nbcon_kthreads_wake();
+	if (ft.legacy_offload)
+		defer_console_output();
+
 	pr_flush(1000, true);
 }
 
@@ -2709,10 +2775,16 @@ void resume_console(void)
  */
 static int console_cpu_notify(unsigned int cpu)
 {
+	struct console_flush_type ft;
+
 	if (!cpuhp_tasks_frozen) {
-		/* If trylock fails, someone else is doing the printing */
-		if (console_trylock())
-			console_unlock();
+		printk_get_console_flush_type(&ft);
+		if (ft.nbcon_atomic)
+			nbcon_atomic_flush_pending();
+		if (ft.legacy_direct) {
+			if (console_trylock())
+				console_unlock();
+		}
 	}
 	return 0;
 }
@@ -2766,36 +2838,6 @@ int is_console_locked(void)
 }
 EXPORT_SYMBOL(is_console_locked);
 
-/*
- * Check if the given console is currently capable and allowed to print
- * records.
- *
- * Requires the console_srcu_read_lock.
- */
-static inline bool console_is_usable(struct console *con)
-{
-	short flags = console_srcu_read_flags(con);
-
-	if (!(flags & CON_ENABLED))
-		return false;
-
-	if ((flags & CON_SUSPENDED))
-		return false;
-
-	if (!con->write)
-		return false;
-
-	/*
-	 * Console drivers may assume that per-cpu resources have been
-	 * allocated. So unless they're explicitly marked as being able to
-	 * cope (CON_ANYTIME) don't call them until this CPU is officially up.
-	 */
-	if (!cpu_online(raw_smp_processor_id()) && !(flags & CON_ANYTIME))
-		return false;
-
-	return true;
-}
-
 static void __console_unlock(void)
 {
 	console_locked = 0;
@@ -2805,30 +2847,31 @@ static void __console_unlock(void)
 #ifdef CONFIG_PRINTK
 
 /*
- * Prepend the message in @pmsg->pbufs->outbuf with a "dropped message". This
- * is achieved by shifting the existing message over and inserting the dropped
- * message.
+ * Prepend the message in @pmsg->pbufs->outbuf. This is achieved by shifting
+ * the existing message over and inserting the scratchbuf message.
  *
- * @pmsg is the printk message to prepend.
- *
- * @dropped is the dropped count to report in the dropped message.
+ * @pmsg is the original printk message.
+ * @fmt is the printf format of the message which will prepend the existing one.
  *
- * If the message text in @pmsg->pbufs->outbuf does not have enough space for
- * the dropped message, the message text will be sufficiently truncated.
+ * If there is not enough space in @pmsg->pbufs->outbuf, the existing
+ * message text will be sufficiently truncated.
  *
  * If @pmsg->pbufs->outbuf is modified, @pmsg->outbuf_len is updated.
  */
-void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
+__printf(2, 3)
+static void console_prepend_message(struct printk_message *pmsg, const char *fmt, ...)
 {
 	struct printk_buffers *pbufs = pmsg->pbufs;
 	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
 	const size_t outbuf_sz = sizeof(pbufs->outbuf);
 	char *scratchbuf = &pbufs->scratchbuf[0];
 	char *outbuf = &pbufs->outbuf[0];
+	va_list args;
 	size_t len;
 
-	len = scnprintf(scratchbuf, scratchbuf_sz,
-		       "** %lu printk messages dropped **\n", dropped);
+	va_start(args, fmt);
+	len = vscnprintf(scratchbuf, scratchbuf_sz, fmt, args);
+	va_end(args);
 
 	/*
 	 * Make sure outbuf is sufficiently large before prepending.
@@ -2851,6 +2894,30 @@ void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
 }
 
 /*
+ * Prepend the message in @pmsg->pbufs->outbuf with a "dropped message".
+ * @pmsg->outbuf_len is updated appropriately.
+ *
+ * @pmsg is the printk message to prepend.
+ *
+ * @dropped is the dropped count to report in the dropped message.
+ */
+void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
+{
+	console_prepend_message(pmsg, "** %lu printk messages dropped **\n", dropped);
+}
+
+/*
+ * Prepend the message in @pmsg->pbufs->outbuf with a "replay message".
+ * @pmsg->outbuf_len is updated appropriately.
+ *
+ * @pmsg is the printk message to prepend.
+ */
+void console_prepend_replay(struct printk_message *pmsg)
+{
+	console_prepend_message(pmsg, "** replaying previous printk message **\n");
+}
+
+/*
  * Read and format the specified record (or a later record if the specified
  * record is not available).
  *
@@ -2916,6 +2983,34 @@ out:
 }
 
 /*
+ * Legacy console printing from printk() caller context does not respect
+ * raw_spinlock/spinlock nesting. For !PREEMPT_RT the lockdep warning is a
+ * false positive. For PREEMPT_RT the false positive condition does not
+ * occur.
+ *
+ * This map is used to temporarily establish LD_WAIT_SLEEP context for the
+ * console write() callback when legacy printing to avoid false positive
+ * lockdep complaints, thus allowing lockdep to continue to function for
+ * real issues.
+ */
+#ifdef CONFIG_PREEMPT_RT
+static inline void printk_legacy_allow_spinlock_enter(void) { }
+static inline void printk_legacy_allow_spinlock_exit(void) { }
+#else
+static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);
+
+static inline void printk_legacy_allow_spinlock_enter(void)
+{
+	lock_map_acquire_try(&printk_legacy_map);
+}
+
+static inline void printk_legacy_allow_spinlock_exit(void)
+{
+	lock_map_release(&printk_legacy_map);
+}
+#endif /* CONFIG_PREEMPT_RT */
+
+/*
  * Used as the printk buffers for non-panic, serialized console printing.
  * This is for legacy (!CON_NBCON) as well as all boot (CON_BOOT) consoles.
  * Its usage requires the console_lock held.
@@ -2964,31 +3059,46 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 		con->dropped = 0;
 	}
 
-	/*
-	 * While actively printing out messages, if another printk()
-	 * were to occur on another CPU, it may wait for this one to
-	 * finish. This task can not be preempted if there is a
-	 * waiter waiting to take over.
-	 *
-	 * Interrupts are disabled because the hand over to a waiter
-	 * must not be interrupted until the hand over is completed
-	 * (@console_waiter is cleared).
-	 */
-	printk_safe_enter_irqsave(flags);
-	console_lock_spinning_enable();
+	/* Write everything out to the hardware. */
 
-	/* Do not trace print latency. */
-	stop_critical_timings();
+	if (force_legacy_kthread() && !panic_in_progress()) {
+		/*
+		 * With forced threading this function is in a task context
+		 * (either legacy kthread or get_init_console_seq()). There
+		 * is no need for concern about printk reentrance, handovers,
+		 * or lockdep complaints.
+		 */
 
-	/* Write everything out to the hardware. */
-	con->write(con, outbuf, pmsg.outbuf_len);
+		con->write(con, outbuf, pmsg.outbuf_len);
+		con->seq = pmsg.seq + 1;
+	} else {
+		/*
+		 * While actively printing out messages, if another printk()
+		 * were to occur on another CPU, it may wait for this one to
+		 * finish. This task can not be preempted if there is a
+		 * waiter waiting to take over.
+		 *
+		 * Interrupts are disabled because the hand over to a waiter
+		 * must not be interrupted until the hand over is completed
+		 * (@console_waiter is cleared).
+		 */
+		printk_safe_enter_irqsave(flags);
+		console_lock_spinning_enable();
 
-	start_critical_timings();
+		/* Do not trace print latency. */
+		stop_critical_timings();
 
-	con->seq = pmsg.seq + 1;
+		printk_legacy_allow_spinlock_enter();
+		con->write(con, outbuf, pmsg.outbuf_len);
+		printk_legacy_allow_spinlock_exit();
 
-	*handover = console_lock_spinning_disable_and_check(cookie);
-	printk_safe_exit_irqrestore(flags);
+		start_critical_timings();
+
+		con->seq = pmsg.seq + 1;
+
+		*handover = console_lock_spinning_disable_and_check(cookie);
+		printk_safe_exit_irqrestore(flags);
+	}
 skip:
 	return true;
 }
@@ -3001,6 +3111,8 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 	return false;
 }
 
+static inline void printk_kthreads_check_locked(void) { }
+
 #endif /* CONFIG_PRINTK */
 
 /*
@@ -3028,6 +3140,7 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
  */
 static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover)
 {
+	struct console_flush_type ft;
 	bool any_usable = false;
 	struct console *con;
 	bool any_progress;
@@ -3039,15 +3152,34 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 	do {
 		any_progress = false;
 
+		printk_get_console_flush_type(&ft);
+
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(con) {
+			short flags = console_srcu_read_flags(con);
+			u64 printk_seq;
 			bool progress;
 
-			if (!console_is_usable(con))
+			/*
+			 * console_flush_all() is only responsible for nbcon
+			 * consoles when the nbcon consoles cannot print via
+			 * their atomic or threaded flushing.
+			 */
+			if ((flags & CON_NBCON) && (ft.nbcon_atomic || ft.nbcon_offload))
+				continue;
+
+			if (!console_is_usable(con, flags, !do_cond_resched))
 				continue;
 			any_usable = true;
 
-			progress = console_emit_next_record(con, handover, cookie);
+			if (flags & CON_NBCON) {
+				progress = nbcon_legacy_emit_next_record(con, handover, cookie,
+									 !do_cond_resched);
+				printk_seq = nbcon_seq_read(con);
+			} else {
+				progress = console_emit_next_record(con, handover, cookie);
+				printk_seq = con->seq;
+			}
 
 			/*
 			 * If a handover has occurred, the SRCU read lock
@@ -3057,8 +3189,8 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 				return false;
 
 			/* Track the next of the highest seq flushed. */
-			if (con->seq > *next_seq)
-				*next_seq = con->seq;
+			if (printk_seq > *next_seq)
+				*next_seq = printk_seq;
 
 			if (!progress)
 				continue;
@@ -3081,19 +3213,7 @@ abandon:
 	return false;
 }
 
-/**
- * console_unlock - unblock the console subsystem from printing
- *
- * Releases the console_lock which the caller holds to block printing of
- * the console subsystem.
- *
- * While the console_lock was held, console output may have been buffered
- * by printk().  If this is the case, console_unlock(); emits
- * the output prior to releasing the lock.
- *
- * console_unlock(); may be called from any context.
- */
-void console_unlock(void)
+static void __console_flush_and_unlock(void)
 {
 	bool do_cond_resched;
 	bool handover;
@@ -3137,6 +3257,29 @@ void console_unlock(void)
 		 */
 	} while (prb_read_valid(prb, next_seq, NULL) && console_trylock());
 }
+
+/**
+ * console_unlock - unblock the legacy console subsystem from printing
+ *
+ * Releases the console_lock which the caller holds to block printing of
+ * the legacy console subsystem.
+ *
+ * While the console_lock was held, console output may have been buffered
+ * by printk(). If this is the case, console_unlock() emits the output on
+ * legacy consoles prior to releasing the lock.
+ *
+ * console_unlock(); may be called from any context.
+ */
+void console_unlock(void)
+{
+	struct console_flush_type ft;
+
+	printk_get_console_flush_type(&ft);
+	if (ft.legacy_direct)
+		__console_flush_and_unlock();
+	else
+		__console_unlock();
+}
 EXPORT_SYMBOL(console_unlock);
 
 /**
@@ -3259,6 +3402,7 @@ static void __console_rewind_all(void)
  */
 void console_flush_on_panic(enum con_flush_mode mode)
 {
+	struct console_flush_type ft;
 	bool handover;
 	u64 next_seq;
 
@@ -3282,7 +3426,13 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	if (mode == CONSOLE_REPLAY_ALL)
 		__console_rewind_all();
 
-	console_flush_all(false, &next_seq, &handover);
+	printk_get_console_flush_type(&ft);
+	if (ft.nbcon_atomic)
+		nbcon_atomic_flush_pending();
+
+	/* Flush legacy consoles once allowed, even when dangerous. */
+	if (legacy_allow_panic_sync)
+		console_flush_all(false, &next_seq, &handover);
 }
 
 /*
@@ -3339,13 +3489,236 @@ EXPORT_SYMBOL(console_stop);
 
 void console_start(struct console *console)
 {
+	struct console_flush_type ft;
+	bool is_nbcon;
+
 	console_list_lock();
 	console_srcu_write_flags(console, console->flags | CON_ENABLED);
+	is_nbcon = console->flags & CON_NBCON;
 	console_list_unlock();
+
+	/*
+	 * Ensure that all SRCU list walks have completed. The related
+	 * printing context must be able to see it is enabled so that
+	 * it is guaranteed to wake up and resume printing.
+	 */
+	synchronize_srcu(&console_srcu);
+
+	printk_get_console_flush_type(&ft);
+	if (is_nbcon && ft.nbcon_offload)
+		nbcon_kthread_wake(console);
+	else if (ft.legacy_offload)
+		defer_console_output();
+
 	__pr_flush(console, 1000, true);
 }
 EXPORT_SYMBOL(console_start);
 
+#ifdef CONFIG_PRINTK
+static int unregister_console_locked(struct console *console);
+
+/* True when system boot is far enough to create printer threads. */
+static bool printk_kthreads_ready __ro_after_init;
+
+static struct task_struct *printk_legacy_kthread;
+
+static bool legacy_kthread_should_wakeup(void)
+{
+	struct console_flush_type ft;
+	struct console *con;
+	bool ret = false;
+	int cookie;
+
+	if (kthread_should_stop())
+		return true;
+
+	printk_get_console_flush_type(&ft);
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(con) {
+		short flags = console_srcu_read_flags(con);
+		u64 printk_seq;
+
+		/*
+		 * The legacy printer thread is only responsible for nbcon
+		 * consoles when the nbcon consoles cannot print via their
+		 * atomic or threaded flushing.
+		 */
+		if ((flags & CON_NBCON) && (ft.nbcon_atomic || ft.nbcon_offload))
+			continue;
+
+		if (!console_is_usable(con, flags, false))
+			continue;
+
+		if (flags & CON_NBCON) {
+			printk_seq = nbcon_seq_read(con);
+		} else {
+			/*
+			 * It is safe to read @seq because only this
+			 * thread context updates @seq.
+			 */
+			printk_seq = con->seq;
+		}
+
+		if (prb_read_valid(prb, printk_seq, NULL)) {
+			ret = true;
+			break;
+		}
+	}
+	console_srcu_read_unlock(cookie);
+
+	return ret;
+}
+
+static int legacy_kthread_func(void *unused)
+{
+	for (;;) {
+		wait_event_interruptible(legacy_wait, legacy_kthread_should_wakeup());
+
+		if (kthread_should_stop())
+			break;
+
+		console_lock();
+		__console_flush_and_unlock();
+	}
+
+	return 0;
+}
+
+static bool legacy_kthread_create(void)
+{
+	struct task_struct *kt;
+
+	lockdep_assert_console_list_lock_held();
+
+	kt = kthread_run(legacy_kthread_func, NULL, "pr/legacy");
+	if (WARN_ON(IS_ERR(kt))) {
+		pr_err("failed to start legacy printing thread\n");
+		return false;
+	}
+
+	printk_legacy_kthread = kt;
+
+	/*
+	 * It is important that console printing threads are scheduled
+	 * shortly after a printk call and with generous runtime budgets.
+	 */
+	sched_set_normal(printk_legacy_kthread, -20);
+
+	return true;
+}
+
+/**
+ * printk_kthreads_shutdown - shutdown all threaded printers
+ *
+ * On system shutdown all threaded printers are stopped. This allows printk
+ * to transition back to atomic printing, thus providing a robust mechanism
+ * for the final shutdown/reboot messages to be output.
+ */
+static void printk_kthreads_shutdown(void)
+{
+	struct console *con;
+
+	console_list_lock();
+	if (printk_kthreads_running) {
+		printk_kthreads_running = false;
+
+		for_each_console(con) {
+			if (con->flags & CON_NBCON)
+				nbcon_kthread_stop(con);
+		}
+
+		/*
+		 * The threads may have been stopped while printing a
+		 * backlog. Flush any records left over.
+		 */
+		nbcon_atomic_flush_pending();
+	}
+	console_list_unlock();
+}
+
+static struct syscore_ops printk_syscore_ops = {
+	.shutdown = printk_kthreads_shutdown,
+};
+
+/*
+ * If appropriate, start nbcon kthreads and set @printk_kthreads_running.
+ * If any kthreads fail to start, those consoles are unregistered.
+ *
+ * Must be called under console_list_lock().
+ */
+static void printk_kthreads_check_locked(void)
+{
+	struct hlist_node *tmp;
+	struct console *con;
+
+	lockdep_assert_console_list_lock_held();
+
+	if (!printk_kthreads_ready)
+		return;
+
+	if (have_legacy_console || have_boot_console) {
+		if (!printk_legacy_kthread &&
+		    force_legacy_kthread() &&
+		    !legacy_kthread_create()) {
+			/*
+			 * All legacy consoles must be unregistered. If there
+			 * are any nbcon consoles, they will set up their own
+			 * kthread.
+			 */
+			hlist_for_each_entry_safe(con, tmp, &console_list, node) {
+				if (con->flags & CON_NBCON)
+					continue;
+
+				unregister_console_locked(con);
+			}
+		}
+	} else if (printk_legacy_kthread) {
+		kthread_stop(printk_legacy_kthread);
+		printk_legacy_kthread = NULL;
+	}
+
+	/*
+	 * Printer threads cannot be started as long as any boot console is
+	 * registered because there is no way to synchronize the hardware
+	 * registers between boot console code and regular console code.
+	 * It can only be known that there will be no new boot consoles when
+	 * an nbcon console is registered.
+	 */
+	if (have_boot_console || !have_nbcon_console) {
+		/* Clear flag in case all nbcon consoles unregistered. */
+		printk_kthreads_running = false;
+		return;
+	}
+
+	if (printk_kthreads_running)
+		return;
+
+	hlist_for_each_entry_safe(con, tmp, &console_list, node) {
+		if (!(con->flags & CON_NBCON))
+			continue;
+
+		if (!nbcon_kthread_create(con))
+			unregister_console_locked(con);
+	}
+
+	printk_kthreads_running = true;
+}
+
+static int __init printk_set_kthreads_ready(void)
+{
+	register_syscore_ops(&printk_syscore_ops);
+
+	console_list_lock();
+	printk_kthreads_ready = true;
+	printk_kthreads_check_locked();
+	console_list_unlock();
+
+	return 0;
+}
+early_initcall(printk_set_kthreads_ready);
+#endif /* CONFIG_PRINTK */
+
 static int __read_mostly keep_bootcon;
 
 static int __init keep_bootcon_setup(char *str)
@@ -3447,19 +3820,21 @@ static void try_enable_default_console(struct console *newcon)
 		newcon->flags |= CON_CONSDEV;
 }
 
-static void console_init_seq(struct console *newcon, bool bootcon_registered)
+/* Return the starting sequence number for a newly registered console. */
+static u64 get_init_console_seq(struct console *newcon, bool bootcon_registered)
 {
 	struct console *con;
 	bool handover;
+	u64 init_seq;
 
 	if (newcon->flags & (CON_PRINTBUFFER | CON_BOOT)) {
 		/* Get a consistent copy of @syslog_seq. */
 		mutex_lock(&syslog_lock);
-		newcon->seq = syslog_seq;
+		init_seq = syslog_seq;
 		mutex_unlock(&syslog_lock);
 	} else {
 		/* Begin with next message added to ringbuffer. */
-		newcon->seq = prb_next_seq(prb);
+		init_seq = prb_next_seq(prb);
 
 		/*
 		 * If any enabled boot consoles are due to be unregistered
@@ -3480,7 +3855,7 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
 			 * Flush all consoles and set the console to start at
 			 * the next unprinted sequence number.
 			 */
-			if (!console_flush_all(true, &newcon->seq, &handover)) {
+			if (!console_flush_all(true, &init_seq, &handover)) {
 				/*
 				 * Flushing failed. Just choose the lowest
 				 * sequence of the enabled boot consoles.
@@ -3493,19 +3868,30 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
 				if (handover)
 					console_lock();
 
-				newcon->seq = prb_next_seq(prb);
+				init_seq = prb_next_seq(prb);
 				for_each_console(con) {
-					if ((con->flags & CON_BOOT) &&
-					    (con->flags & CON_ENABLED) &&
-					    con->seq < newcon->seq) {
-						newcon->seq = con->seq;
+					u64 seq;
+
+					if (!(con->flags & CON_BOOT) ||
+					    !(con->flags & CON_ENABLED)) {
+						continue;
 					}
+
+					if (con->flags & CON_NBCON)
+						seq = nbcon_seq_read(con);
+					else
+						seq = con->seq;
+
+					if (seq < init_seq)
+						init_seq = seq;
 				}
 			}
 
 			console_unlock();
 		}
 	}
+
+	return init_seq;
 }
 
 #define console_first()				\
@@ -3534,9 +3920,12 @@ static int unregister_console_locked(struct console *console);
  */
 void register_console(struct console *newcon)
 {
-	struct console *con;
+	bool use_device_lock = (newcon->flags & CON_NBCON) && newcon->write_atomic;
 	bool bootcon_registered = false;
 	bool realcon_registered = false;
+	struct console *con;
+	unsigned long flags;
+	u64 init_seq;
 	int err;
 
 	console_list_lock();
@@ -3614,10 +4003,31 @@ void register_console(struct console *newcon)
 	}
 
 	newcon->dropped = 0;
-	console_init_seq(newcon, bootcon_registered);
+	init_seq = get_init_console_seq(newcon, bootcon_registered);
+
+	if (newcon->flags & CON_NBCON) {
+		have_nbcon_console = true;
+		nbcon_seq_force(newcon, init_seq);
+	} else {
+		have_legacy_console = true;
+		newcon->seq = init_seq;
+	}
+
+	if (newcon->flags & CON_BOOT)
+		have_boot_console = true;
 
-	if (newcon->flags & CON_NBCON)
-		nbcon_init(newcon);
+	/*
+	 * If another context is actively using the hardware of this new
+	 * console, it will not be aware of the nbcon synchronization. This
+	 * is a risk that two contexts could access the hardware
+	 * simultaneously if this new console is used for atomic printing
+	 * and the other context is still using the hardware.
+	 *
+	 * Use the driver synchronization to ensure that the hardware is not
+	 * in use while this new console transitions to being registered.
+	 */
+	if (use_device_lock)
+		newcon->device_lock(newcon, &flags);
 
 	/*
 	 * Put this console in the list - keep the
@@ -3643,6 +4053,10 @@ void register_console(struct console *newcon)
 	 * register_console() completes.
 	 */
 
+	/* This new console is now registered. */
+	if (use_device_lock)
+		newcon->device_unlock(newcon, flags);
+
 	console_sysfs_notify();
 
 	/*
@@ -3663,6 +4077,9 @@ void register_console(struct console *newcon)
 				unregister_console_locked(con);
 		}
 	}
+
+	/* Changed console list, may require printer threads to start/stop. */
+	printk_kthreads_check_locked();
 unlock:
 	console_list_unlock();
 }
@@ -3671,6 +4088,12 @@ EXPORT_SYMBOL(register_console);
 /* Must be called under console_list_lock(). */
 static int unregister_console_locked(struct console *console)
 {
+	bool use_device_lock = (console->flags & CON_NBCON) && console->write_atomic;
+	bool found_legacy_con = false;
+	bool found_nbcon_con = false;
+	bool found_boot_con = false;
+	unsigned long flags;
+	struct console *c;
 	int res;
 
 	lockdep_assert_console_list_lock_held();
@@ -3683,14 +4106,29 @@ static int unregister_console_locked(struct console *console)
 	if (res > 0)
 		return 0;
 
+	if (!console_is_registered_locked(console))
+		res = -ENODEV;
+	else if (console_is_usable(console, console->flags, true))
+		__pr_flush(console, 1000, true);
+
 	/* Disable it unconditionally */
 	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
 
-	if (!console_is_registered_locked(console))
-		return -ENODEV;
+	if (res < 0)
+		return res;
+
+	/*
+	 * Use the driver synchronization to ensure that the hardware is not
+	 * in use while this console transitions to being unregistered.
+	 */
+	if (use_device_lock)
+		console->device_lock(console, &flags);
 
 	hlist_del_init_rcu(&console->node);
 
+	if (use_device_lock)
+		console->device_unlock(console, flags);
+
 	/*
 	 * <HISTORICAL>
 	 * If this isn't the last console and it has CON_CONSDEV set, we
@@ -3718,6 +4156,29 @@ static int unregister_console_locked(struct console *console)
 	if (console->exit)
 		res = console->exit(console);
 
+	/*
+	 * With this console gone, the global flags tracking registered
+	 * console types may have changed. Update them.
+	 */
+	for_each_console(c) {
+		if (c->flags & CON_BOOT)
+			found_boot_con = true;
+
+		if (c->flags & CON_NBCON)
+			found_nbcon_con = true;
+		else
+			found_legacy_con = true;
+	}
+	if (!found_boot_con)
+		have_boot_console = found_boot_con;
+	if (!found_legacy_con)
+		have_legacy_console = found_legacy_con;
+	if (!found_nbcon_con)
+		have_nbcon_console = found_nbcon_con;
+
+	/* Changed console list, may require printer threads to start/stop. */
+	printk_kthreads_check_locked();
+
 	return res;
 }
 
@@ -3864,6 +4325,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 {
 	unsigned long timeout_jiffies = msecs_to_jiffies(timeout_ms);
 	unsigned long remaining_jiffies = timeout_jiffies;
+	struct console_flush_type ft;
 	struct console *c;
 	u64 last_diff = 0;
 	u64 printk_seq;
@@ -3872,13 +4334,22 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 	u64 diff;
 	u64 seq;
 
+	/* Sorry, pr_flush() will not work this early. */
+	if (system_state < SYSTEM_SCHEDULING)
+		return false;
+
 	might_sleep();
 
 	seq = prb_next_reserve_seq(prb);
 
 	/* Flush the consoles so that records up to @seq are printed. */
-	console_lock();
-	console_unlock();
+	printk_get_console_flush_type(&ft);
+	if (ft.nbcon_atomic)
+		nbcon_atomic_flush_pending();
+	if (ft.legacy_direct) {
+		console_lock();
+		console_unlock();
+	}
 
 	for (;;) {
 		unsigned long begin_jiffies;
@@ -3891,6 +4362,12 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 		 * console->seq. Releasing console_lock flushes more
 		 * records in case @seq is still not printed on all
 		 * usable consoles.
+		 *
+		 * Holding the console_lock is not necessary if there
+		 * are no legacy or boot consoles. However, such a
+		 * console could register at any time. Always hold the
+		 * console_lock as a precaution rather than
+		 * synchronizing against register_console().
 		 */
 		console_lock();
 
@@ -3906,8 +4383,10 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 			 * that they make forward progress, so only increment
 			 * @diff for usable consoles.
 			 */
-			if (!console_is_usable(c))
+			if (!console_is_usable(c, flags, true) &&
+			    !console_is_usable(c, flags, false)) {
 				continue;
+			}
 
 			if (flags & CON_NBCON) {
 				printk_seq = nbcon_seq_read(c);
@@ -3975,9 +4454,13 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
 	int pending = this_cpu_xchg(printk_pending, 0);
 
 	if (pending & PRINTK_PENDING_OUTPUT) {
-		/* If trylock fails, someone else is doing the printing */
-		if (console_trylock())
-			console_unlock();
+		if (force_legacy_kthread()) {
+			if (printk_legacy_kthread)
+				wake_up_interruptible(&legacy_wait);
+		} else {
+			if (console_trylock())
+				console_unlock();
+		}
 	}
 
 	if (pending & PRINTK_PENDING_WAKEUP)
@@ -4383,8 +4866,17 @@ EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
  */
 void console_try_replay_all(void)
 {
+	struct console_flush_type ft;
+
+	printk_get_console_flush_type(&ft);
 	if (console_trylock()) {
 		__console_rewind_all();
+		if (ft.nbcon_atomic)
+			nbcon_atomic_flush_pending();
+		if (ft.nbcon_offload)
+			nbcon_kthreads_wake();
+		if (ft.legacy_offload)
+			defer_console_output();
 		/* Consoles are flushed as part of console_unlock(). */
 		console_unlock();
 	}