Merge tag 'printk-for-5.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk kernel thread revert from Petr Mladek:
 "Revert printk console kthreads.

  The testing of 5.19 release candidates revealed issues that did not
  happen when all consoles were serialized using the console semaphore.

  More time is needed to check expectations of the existing console
  drivers and be confident that they can be safely used in parallel"

* tag 'printk-for-5.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  Revert "printk: add functions to prefer direct printing"
  Revert "printk: add kthread console printers"
  Revert "printk: extend console_lock for per-console locking"
  Revert "printk: remove @console_locked"
  Revert "printk: Block console kthreads when direct printing will be required"
  Revert "printk: Wait for the global console lock when the system is going down"

drivers/tty/sysrq.c

@@ -581,7 +581,6 @@ void __handle_sysrq(int key, bool check_mask)
 
 	rcu_sysrq_start();
 	rcu_read_lock();
-	printk_prefer_direct_enter();
 	/*
 	 * Raise the apparent loglevel to maximum so that the sysrq header
 	 * is shown to provide the user with positive feedback.  We do not
@@ -623,7 +622,6 @@ void __handle_sysrq(int key, bool check_mask)
 		pr_cont("\n");
 		console_loglevel = orig_log_level;
 	}
-	printk_prefer_direct_exit();
 	rcu_read_unlock();
 	rcu_sysrq_end();
 

include/linux/console.h

@@ -16,7 +16,6 @@
 
 #include <linux/atomic.h>
 #include <linux/types.h>
-#include <linux/mutex.h>
 
 struct vc_data;
 struct console_font_op;
@@ -154,22 +153,6 @@ struct console {
 	uint	ospeed;
 	u64	seq;
 	unsigned long dropped;
-	struct task_struct *thread;
-	bool	blocked;
-
-	/*
-	 * The per-console lock is used by printing kthreads to synchronize
-	 * this console with callers of console_lock(). This is necessary in
-	 * order to allow printing kthreads to run in parallel to each other,
-	 * while each safely accessing the @blocked field and synchronizing
-	 * against direct printing via console_lock/console_unlock.
-	 *
-	 * Note: For synchronizing against direct printing via
-	 *       console_trylock/console_unlock, see the static global
-	 *       variable @console_kthreads_active.
-	 */
-	struct mutex lock;
-
 	void	*data;
 	struct	 console *next;
 };

include/linux/printk.h

@@ -169,11 +169,7 @@ extern void __printk_safe_exit(void);
 #define printk_deferred_enter __printk_safe_enter
 #define printk_deferred_exit __printk_safe_exit
 
-extern void printk_prefer_direct_enter(void);
-extern void printk_prefer_direct_exit(void);
-
 extern bool pr_flush(int timeout_ms, bool reset_on_progress);
-extern void try_block_console_kthreads(int timeout_ms);
 
 /*
  * Please don't use printk_ratelimit(), because it shares ratelimiting state
@@ -225,23 +221,11 @@ static inline void printk_deferred_exit(void)
 {
 }
 
-static inline void printk_prefer_direct_enter(void)
-{
-}
-
-static inline void printk_prefer_direct_exit(void)
-{
-}
-
 static inline bool pr_flush(int timeout_ms, bool reset_on_progress)
 {
 	return true;
 }
 
-static inline void try_block_console_kthreads(int timeout_ms)
-{
-}
-
 static inline int printk_ratelimit(void)
 {
 	return 0;

kernel/hung_task.c

@@ -127,8 +127,6 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 	 * complain:
 	 */
 	if (sysctl_hung_task_warnings) {
-		printk_prefer_direct_enter();
-
 		if (sysctl_hung_task_warnings > 0)
 			sysctl_hung_task_warnings--;
 		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
@@ -144,8 +142,6 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
 		if (sysctl_hung_task_all_cpu_backtrace)
 			hung_task_show_all_bt = true;
-
-		printk_prefer_direct_exit();
 	}
 
 	touch_nmi_watchdog();
@@ -208,17 +204,12 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 	}
  unlock:
 	rcu_read_unlock();
-	if (hung_task_show_lock) {
-		printk_prefer_direct_enter();
+	if (hung_task_show_lock)
 		debug_show_all_locks();
-		printk_prefer_direct_exit();
-	}
 
 	if (hung_task_show_all_bt) {
 		hung_task_show_all_bt = false;
-		printk_prefer_direct_enter();
 		trigger_all_cpu_backtrace();
-		printk_prefer_direct_exit();
 	}
 
 	if (hung_task_call_panic)

kernel/panic.c

@@ -297,7 +297,6 @@ void panic(const char *fmt, ...)
 		 * unfortunately means it may not be hardened to work in a
 		 * panic situation.
 		 */
-		try_block_console_kthreads(10000);
 		smp_send_stop();
 	} else {
 		/*
@@ -305,7 +304,6 @@ void panic(const char *fmt, ...)
 		 * kmsg_dump, we will need architecture dependent extra
 		 * works in addition to stopping other CPUs.
 		 */
-		try_block_console_kthreads(10000);
 		crash_smp_send_stop();
 	}
 
@@ -605,8 +603,6 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 {
 	disable_trace_on_warning();
 
-	printk_prefer_direct_enter();
-
 	if (file)
 		pr_warn("WARNING: CPU: %d PID: %d at %s:%d %pS\n",
 			raw_smp_processor_id(), current->pid, file, line,
@@ -636,8 +632,6 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 
 	/* Just a warning, don't kill lockdep. */
 	add_taint(taint, LOCKDEP_STILL_OK);
-
-	printk_prefer_direct_exit();
 }
 
 #ifndef __WARN_FLAGS

kernel/printk/internal.h

@@ -20,8 +20,6 @@ enum printk_info_flags {
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
 
-extern bool block_console_kthreads;
-
 __printf(4, 0)
 int vprintk_store(int facility, int level,
 		  const struct dev_printk_info *dev_info,

kernel/printk/printk.c

@@ -223,36 +223,6 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 /* Number of registered extended console drivers. */
 static int nr_ext_console_drivers;
 
-/*
- * Used to synchronize printing kthreads against direct printing via
- * console_trylock/console_unlock.
- *
- * Values:
- * -1 = console kthreads atomically blocked (via global trylock)
- *  0 = no kthread printing, console not locked (via trylock)
- * >0 = kthread(s) actively printing
- *
- * Note: For synchronizing against direct printing via
- *       console_lock/console_unlock, see the @lock variable in
- *       struct console.
- */
-static atomic_t console_kthreads_active = ATOMIC_INIT(0);
-
-#define console_kthreads_atomic_tryblock() \
-	(atomic_cmpxchg(&console_kthreads_active, 0, -1) == 0)
-#define console_kthreads_atomic_unblock() \
-	atomic_cmpxchg(&console_kthreads_active, -1, 0)
-#define console_kthreads_atomically_blocked() \
-	(atomic_read(&console_kthreads_active) == -1)
-
-#define console_kthread_printing_tryenter() \
-	atomic_inc_unless_negative(&console_kthreads_active)
-#define console_kthread_printing_exit() \
-	atomic_dec(&console_kthreads_active)
-
-/* Block console kthreads to avoid processing new messages. */
-bool block_console_kthreads;
-
 /*
  * Helper macros to handle lockdep when locking/unlocking console_sem. We use
  * macros instead of functions so that _RET_IP_ contains useful information.
@@ -301,49 +271,14 @@ static bool panic_in_progress(void)
 }
 
 /*
- * Tracks whether kthread printers are all blocked. A value of true implies
- * that the console is locked via console_lock() or the console is suspended.
- * Writing to this variable requires holding @console_sem.
+ * This is used for debugging the mess that is the VT code by
+ * keeping track if we have the console semaphore held. It's
+ * definitely not the perfect debug tool (we don't know if _WE_
+ * hold it and are racing, but it helps tracking those weird code
+ * paths in the console code where we end up in places I want
+ * locked without the console semaphore held).
  */
-static bool console_kthreads_blocked;
-
-/*
- * Block all kthread printers from a schedulable context.
- *
- * Requires holding @console_sem.
- */
-static void console_kthreads_block(void)
-{
-	struct console *con;
-
-	for_each_console(con) {
-		mutex_lock(&con->lock);
-		con->blocked = true;
-		mutex_unlock(&con->lock);
-	}
-
-	console_kthreads_blocked = true;
-}
-
-/*
- * Unblock all kthread printers from a schedulable context.
- *
- * Requires holding @console_sem.
- */
-static void console_kthreads_unblock(void)
-{
-	struct console *con;
-
-	for_each_console(con) {
-		mutex_lock(&con->lock);
-		con->blocked = false;
-		mutex_unlock(&con->lock);
-	}
-
-	console_kthreads_blocked = false;
-}
-
-static int console_suspended;
+static int console_locked, console_suspended;
 
 /*
  *	Array of consoles built from command line options (console=)
@@ -426,75 +361,7 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 /* syslog_lock protects syslog_* variables and write access to clear_seq. */
 static DEFINE_MUTEX(syslog_lock);
 
-/*
- * A flag to signify if printk_activate_kthreads() has already started the
- * kthread printers. If true, any later registered consoles must start their
- * own kthread directly. The flag is write protected by the console_lock.
- */
-static bool printk_kthreads_available;
-
 #ifdef CONFIG_PRINTK
-static atomic_t printk_prefer_direct = ATOMIC_INIT(0);
-
-/**
- * printk_prefer_direct_enter - cause printk() calls to attempt direct
- *                              printing to all enabled consoles
- *
- * Since it is not possible to call into the console printing code from any
- * context, there is no guarantee that direct printing will occur.
- *
- * This globally effects all printk() callers.
- *
- * Context: Any context.
- */
-void printk_prefer_direct_enter(void)
-{
-	atomic_inc(&printk_prefer_direct);
-}
-
-/**
- * printk_prefer_direct_exit - restore printk() behavior
- *
- * Context: Any context.
- */
-void printk_prefer_direct_exit(void)
-{
-	WARN_ON(atomic_dec_if_positive(&printk_prefer_direct) < 0);
-}
-
-/*
- * Calling printk() always wakes kthread printers so that they can
- * flush the new message to their respective consoles. Also, if direct
- * printing is allowed, printk() tries to flush the messages directly.
- *
- * Direct printing is allowed in situations when the kthreads
- * are not available or the system is in a problematic state.
- *
- * See the implementation about possible races.
- */
-static inline bool allow_direct_printing(void)
-{
-	/*
-	 * Checking kthread availability is a possible race because the
-	 * kthread printers can become permanently disabled during runtime.
-	 * However, doing that requires holding the console_lock, so any
-	 * pending messages will be direct printed by console_unlock().
-	 */
-	if (!printk_kthreads_available)
-		return true;
-
-	/*
-	 * Prefer direct printing when the system is in a problematic state.
-	 * The context that sets this state will always see the updated value.
-	 * The other contexts do not care. Anyway, direct printing is just a
-	 * best effort. The direct output is only possible when console_lock
-	 * is not already taken and no kthread printers are actively printing.
-	 */
-	return (system_state > SYSTEM_RUNNING ||
-		oops_in_progress ||
-		atomic_read(&printk_prefer_direct));
-}
-
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
@@ -2385,10 +2252,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched && allow_direct_printing()) {
+	if (!in_sched) {
 		/*
 		 * The caller may be holding system-critical or
-		 * timing-sensitive locks. Disable preemption during direct
+		 * timing-sensitive locks. Disable preemption during
 		 * printing of all remaining records to all consoles so that
 		 * this context can return as soon as possible. Hopefully
 		 * another printk() caller will take over the printing.
@@ -2431,8 +2298,6 @@ EXPORT_SYMBOL(_printk);
 
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);
 
-static void printk_start_kthread(struct console *con);
-
 #else /* CONFIG_PRINTK */
 
 #define CONSOLE_LOG_MAX		0
@@ -2466,8 +2331,6 @@ static void call_console_driver(struct console *con, const char *text, size_t le
 }
 static bool suppress_message_printing(int level) { return false; }
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }
-static void printk_start_kthread(struct console *con) { }
-static bool allow_direct_printing(void) { return true; }
 
 #endif /* CONFIG_PRINTK */
 
@@ -2686,14 +2549,6 @@ static int console_cpu_notify(unsigned int cpu)
 		/* If trylock fails, someone else is doing the printing */
 		if (console_trylock())
 			console_unlock();
-		else {
-			/*
-			 * If a new CPU comes online, the conditions for
-			 * printer_should_wake() may have changed for some
-			 * kthread printer with !CON_ANYTIME.
-			 */
-			wake_up_klogd();
-		}
 	}
 	return 0;
 }
@@ -2713,7 +2568,7 @@ void console_lock(void)
 	down_console_sem();
 	if (console_suspended)
 		return;
-	console_kthreads_block();
+	console_locked = 1;
 	console_may_schedule = 1;
 }
 EXPORT_SYMBOL(console_lock);
@@ -2734,30 +2589,15 @@ int console_trylock(void)
 		up_console_sem();
 		return 0;
 	}
-	if (!console_kthreads_atomic_tryblock()) {
-		up_console_sem();
-		return 0;
-	}
+	console_locked = 1;
 	console_may_schedule = 0;
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
 
-/*
- * This is used to help to make sure that certain paths within the VT code are
- * running with the console lock held. It is definitely not the perfect debug
- * tool (it is not known if the VT code is the task holding the console lock),
- * but it helps tracking those weird code paths in the console code such as
- * when the console is suspended: where the console is not locked but no
- * console printing may occur.
- *
- * Note: This returns true when the console is suspended but is not locked.
- *       This is intentional because the VT code must consider that situation
- *       the same as if the console was locked.
- */
 int is_console_locked(void)
 {
-	return (console_kthreads_blocked || atomic_read(&console_kthreads_active));
+	return console_locked;
 }
 EXPORT_SYMBOL(is_console_locked);
 
@@ -2780,9 +2620,18 @@ static bool abandon_console_lock_in_panic(void)
 	return atomic_read(&panic_cpu) != raw_smp_processor_id();
 }
 
-static inline bool __console_is_usable(short flags)
+/*
+ * Check if the given console is currently capable and allowed to print
+ * records.
+ *
+ * Requires the console_lock.
+ */
+static inline bool console_is_usable(struct console *con)
 {
-	if (!(flags & CON_ENABLED))
+	if (!(con->flags & CON_ENABLED))
+		return false;
+
+	if (!con->write)
 		return false;
 
 	/*
@@ -2791,43 +2640,15 @@ static inline bool __console_is_usable(short flags)
 	 * cope (CON_ANYTIME) don't call them until this CPU is officially up.
 	 */
 	if (!cpu_online(raw_smp_processor_id()) &&
-	    !(flags & CON_ANYTIME))
+	    !(con->flags & CON_ANYTIME))
 		return false;
 
 	return true;
 }
 
-/*
- * Check if the given console is currently capable and allowed to print
- * records.
- *
- * Requires holding the console_lock.
- */
-static inline bool console_is_usable(struct console *con)
-{
-	if (!con->write)
-		return false;
-
-	return __console_is_usable(con->flags);
-}
-
 static void __console_unlock(void)
 {
-	/*
-	 * Depending on whether console_lock() or console_trylock() was used,
-	 * appropriately allow the kthread printers to continue.
-	 */
-	if (console_kthreads_blocked)
-		console_kthreads_unblock();
-	else
-		console_kthreads_atomic_unblock();
-
-	/*
-	 * New records may have arrived while the console was locked.
-	 * Wake the kthread printers to print them.
-	 */
-	wake_up_klogd();
-
+	console_locked = 0;
 	up_console_sem();
 }
 
@@ -2845,19 +2666,17 @@ static void __console_unlock(void)
  *
  * @handover will be set to true if a printk waiter has taken over the
  * console_lock, in which case the caller is no longer holding the
- * console_lock. Otherwise it is set to false. A NULL pointer may be provided
- * to disable allowing the console_lock to be taken over by a printk waiter.
+ * console_lock. Otherwise it is set to false.
  *
  * Returns false if the given console has no next record to print, otherwise
  * true.
  *
- * Requires the console_lock if @handover is non-NULL.
- * Requires con->lock otherwise.
+ * Requires the console_lock.
  */
-static bool __console_emit_next_record(struct console *con, char *text, char *ext_text,
-				       char *dropped_text, bool *handover)
+static bool console_emit_next_record(struct console *con, char *text, char *ext_text,
+				     char *dropped_text, bool *handover)
 {
-	static atomic_t panic_console_dropped = ATOMIC_INIT(0);
+	static int panic_console_dropped;
 	struct printk_info info;
 	struct printk_record r;
 	unsigned long flags;
@@ -2866,8 +2685,7 @@ static bool __console_emit_next_record(struct console *con, char *text, char *ex
 
 	prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX);
 
-	if (handover)
-		*handover = false;
+	*handover = false;
 
 	if (!prb_read_valid(prb, con->seq, &r))
 		return false;
@@ -2875,8 +2693,7 @@ static bool __console_emit_next_record(struct console *con, char *text, char *ex
 	if (con->seq != r.info->seq) {
 		con->dropped += r.info->seq - con->seq;
 		con->seq = r.info->seq;
-		if (panic_in_progress() &&
-		    atomic_fetch_inc_relaxed(&panic_console_dropped) > 10) {
+		if (panic_in_progress() && panic_console_dropped++ > 10) {
 			suppress_panic_printk = 1;
 			pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
 		}
@@ -2898,61 +2715,31 @@ static bool __console_emit_next_record(struct console *con, char *text, char *ex
 		len = record_print_text(&r, console_msg_format & MSG_FORMAT_SYSLOG, printk_time);
 	}
 
-	if (handover) {
-		/*
-		 * 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();
-
-		/* don't trace irqsoff print latency */
-		stop_critical_timings();
-	}
+	/*
+	 * 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();
 
+	stop_critical_timings();	/* don't trace print latency */
 	call_console_driver(con, write_text, len, dropped_text);
+	start_critical_timings();
 
 	con->seq++;
 
-	if (handover) {
-		start_critical_timings();
-		*handover = console_lock_spinning_disable_and_check();
-		printk_safe_exit_irqrestore(flags);
-	}
+	*handover = console_lock_spinning_disable_and_check();
+	printk_safe_exit_irqrestore(flags);
 skip:
 	return true;
 }
 
-/*
- * Print a record for a given console, but allow another printk() caller to
- * take over the console_lock and continue printing.
- *
- * Requires the console_lock, but depending on @handover after the call, the
- * caller may no longer have the console_lock.
- *
- * See __console_emit_next_record() for argument and return details.
- */
-static bool console_emit_next_record_transferable(struct console *con, char *text, char *ext_text,
-						  char *dropped_text, bool *handover)
-{
-	/*
-	 * Handovers are only supported if threaded printers are atomically
-	 * blocked. The context taking over the console_lock may be atomic.
-	 */
-	if (!console_kthreads_atomically_blocked()) {
-		*handover = false;
-		handover = NULL;
-	}
-
-	return __console_emit_next_record(con, text, ext_text, dropped_text, handover);
-}
-
 /*
  * Print out all remaining records to all consoles.
  *
@@ -2971,8 +2758,8 @@ static bool console_emit_next_record_transferable(struct console *con, char *tex
  * were flushed to all usable consoles. A returned false informs the caller
  * that everything was not flushed (either there were no usable consoles or
  * another context has taken over printing or it is a panic situation and this
- * is not the panic CPU or direct printing is not preferred). Regardless the
- * reason, the caller should assume it is not useful to immediately try again.
+ * is not the panic CPU). Regardless the reason, the caller should assume it
+ * is not useful to immediately try again.
  *
  * Requires the console_lock.
  */
@@ -2989,10 +2776,6 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 	*handover = false;
 
 	do {
-		/* Let the kthread printers do the work if they can. */
-		if (!allow_direct_printing())
-			return false;
-
 		any_progress = false;
 
 		for_each_console(con) {
@@ -3004,11 +2787,13 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 
 			if (con->flags & CON_EXTENDED) {
 				/* Extended consoles do not print "dropped messages". */
-				progress = console_emit_next_record_transferable(con, &text[0],
-								&ext_text[0], NULL, handover);
+				progress = console_emit_next_record(con, &text[0],
+								    &ext_text[0], NULL,
+								    handover);
 			} else {
-				progress = console_emit_next_record_transferable(con, &text[0],
-								NULL, &dropped_text[0], handover);
+				progress = console_emit_next_record(con, &text[0],
+								    NULL, &dropped_text[0],
+								    handover);
 			}
 			if (*handover)
 				return false;
@@ -3123,13 +2908,10 @@ void console_unblank(void)
 	if (oops_in_progress) {
 		if (down_trylock_console_sem() != 0)
 			return;
-		if (!console_kthreads_atomic_tryblock()) {
-			up_console_sem();
-			return;
-		}
 	} else
 		console_lock();
 
+	console_locked = 1;
 	console_may_schedule = 0;
 	for_each_console(c)
 		if ((c->flags & CON_ENABLED) && c->unblank)
@@ -3408,10 +3190,6 @@ void register_console(struct console *newcon)
 		nr_ext_console_drivers++;
 
 	newcon->dropped = 0;
-	newcon->thread = NULL;
-	newcon->blocked = true;
-	mutex_init(&newcon->lock);
-
 	if (newcon->flags & CON_PRINTBUFFER) {
 		/* Get a consistent copy of @syslog_seq. */
 		mutex_lock(&syslog_lock);
@@ -3421,10 +3199,6 @@ void register_console(struct console *newcon)
 		/* Begin with next message. */
 		newcon->seq = prb_next_seq(prb);
 	}
-
-	if (printk_kthreads_available)
-		printk_start_kthread(newcon);
-
 	console_unlock();
 	console_sysfs_notify();
 
@@ -3451,7 +3225,6 @@ EXPORT_SYMBOL(register_console);
 
 int unregister_console(struct console *console)
 {
-	struct task_struct *thd;
 	struct console *con;
 	int res;
 
@@ -3492,20 +3265,7 @@ int unregister_console(struct console *console)
 		console_drivers->flags |= CON_CONSDEV;
 
 	console->flags &= ~CON_ENABLED;
-
-	/*
-	 * console->thread can only be cleared under the console lock. But
-	 * stopping the thread must be done without the console lock. The
-	 * task that clears @thread is the task that stops the kthread.
-	 */
-	thd = console->thread;
-	console->thread = NULL;
-
 	console_unlock();
-
-	if (thd)
-		kthread_stop(thd);
-
 	console_sysfs_notify();
 
 	if (console->exit)
@@ -3601,20 +3361,6 @@ static int __init printk_late_init(void)
 }
 late_initcall(printk_late_init);
 
-static int __init printk_activate_kthreads(void)
-{
-	struct console *con;
-
-	console_lock();
-	printk_kthreads_available = true;
-	for_each_console(con)
-		printk_start_kthread(con);
-	console_unlock();
-
-	return 0;
-}
-early_initcall(printk_activate_kthreads);
-
 #if defined CONFIG_PRINTK
 /* If @con is specified, only wait for that console. Otherwise wait for all. */
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress)
@@ -3689,209 +3435,11 @@ bool pr_flush(int timeout_ms, bool reset_on_progress)
 }
 EXPORT_SYMBOL(pr_flush);
 
-static void __printk_fallback_preferred_direct(void)
-{
-	printk_prefer_direct_enter();
-	pr_err("falling back to preferred direct printing\n");
-	printk_kthreads_available = false;
-}
-
-/*
- * Enter preferred direct printing, but never exit. Mark console threads as
- * unavailable. The system is then forever in preferred direct printing and
- * any printing threads will exit.
- *
- * Must *not* be called under console_lock. Use
- * __printk_fallback_preferred_direct() if already holding console_lock.
- */
-static void printk_fallback_preferred_direct(void)
-{
-	console_lock();
-	__printk_fallback_preferred_direct();
-	console_unlock();
-}
-
-/*
- * Print a record for a given console, not allowing another printk() caller
- * to take over. This is appropriate for contexts that do not have the
- * console_lock.
- *
- * See __console_emit_next_record() for argument and return details.
- */
-static bool console_emit_next_record(struct console *con, char *text, char *ext_text,
-				     char *dropped_text)
-{
-	return __console_emit_next_record(con, text, ext_text, dropped_text, NULL);
-}
-
-static bool printer_should_wake(struct console *con, u64 seq)
-{
-	short flags;
-
-	if (kthread_should_stop() || !printk_kthreads_available)
-		return true;
-
-	if (con->blocked ||
-	    console_kthreads_atomically_blocked() ||
-	    block_console_kthreads ||
-	    system_state > SYSTEM_RUNNING ||
-	    oops_in_progress) {
-		return false;
-	}
-
-	/*
-	 * This is an unsafe read from con->flags, but a false positive is
-	 * not a problem. Worst case it would allow the printer to wake up
-	 * although it is disabled. But the printer will notice that when
-	 * attempting to print and instead go back to sleep.
-	 */
-	flags = data_race(READ_ONCE(con->flags));
-
-	if (!__console_is_usable(flags))
-		return false;
-
-	return prb_read_valid(prb, seq, NULL);
-}
-
-static int printk_kthread_func(void *data)
-{
-	struct console *con = data;
-	char *dropped_text = NULL;
-	char *ext_text = NULL;
-	u64 seq = 0;
-	char *text;
-	int error;
-
-	text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL);
-	if (!text) {
-		con_printk(KERN_ERR, con, "failed to allocate text buffer\n");
-		printk_fallback_preferred_direct();
-		goto out;
-	}
-
-	if (con->flags & CON_EXTENDED) {
-		ext_text = kmalloc(CONSOLE_EXT_LOG_MAX, GFP_KERNEL);
-		if (!ext_text) {
-			con_printk(KERN_ERR, con, "failed to allocate ext_text buffer\n");
-			printk_fallback_preferred_direct();
-			goto out;
-		}
-	} else {
-		dropped_text = kmalloc(DROPPED_TEXT_MAX, GFP_KERNEL);
-		if (!dropped_text) {
-			con_printk(KERN_ERR, con, "failed to allocate dropped_text buffer\n");
-			printk_fallback_preferred_direct();
-			goto out;
-		}
-	}
-
-	con_printk(KERN_INFO, con, "printing thread started\n");
-
-	for (;;) {
-		/*
-		 * Guarantee this task is visible on the waitqueue before
-		 * checking the wake condition.
-		 *
-		 * The full memory barrier within set_current_state() of
-		 * prepare_to_wait_event() pairs with the full memory barrier
-		 * within wq_has_sleeper().
-		 *
-		 * This pairs with __wake_up_klogd:A.
-		 */
-		error = wait_event_interruptible(log_wait,
-				printer_should_wake(con, seq)); /* LMM(printk_kthread_func:A) */
-
-		if (kthread_should_stop() || !printk_kthreads_available)
-			break;
-
-		if (error)
-			continue;
-
-		error = mutex_lock_interruptible(&con->lock);
-		if (error)
-			continue;
-
-		if (con->blocked ||
-		    !console_kthread_printing_tryenter()) {
-			/* Another context has locked the console_lock. */
-			mutex_unlock(&con->lock);
-			continue;
-		}
-
-		/*
-		 * Although this context has not locked the console_lock, it
-		 * is known that the console_lock is not locked and it is not
-		 * possible for any other context to lock the console_lock.
-		 * Therefore it is safe to read con->flags.
-		 */
-
-		if (!__console_is_usable(con->flags)) {
-			console_kthread_printing_exit();
-			mutex_unlock(&con->lock);
-			continue;
-		}
-
-		/*
-		 * Even though the printk kthread is always preemptible, it is
-		 * still not allowed to call cond_resched() from within
-		 * console drivers. The task may become non-preemptible in the
-		 * console driver call chain. For example, vt_console_print()
-		 * takes a spinlock and then can call into fbcon_redraw(),
-		 * which can conditionally invoke cond_resched().
-		 */
-		console_may_schedule = 0;
-		console_emit_next_record(con, text, ext_text, dropped_text);
-
-		seq = con->seq;
-
-		console_kthread_printing_exit();
-
-		mutex_unlock(&con->lock);
-	}
-
-	con_printk(KERN_INFO, con, "printing thread stopped\n");
-out:
-	kfree(dropped_text);
-	kfree(ext_text);
-	kfree(text);
-
-	console_lock();
-	/*
-	 * If this kthread is being stopped by another task, con->thread will
-	 * already be NULL. That is fine. The important thing is that it is
-	 * NULL after the kthread exits.
-	 */
-	con->thread = NULL;
-	console_unlock();
-
-	return 0;
-}
-
-/* Must be called under console_lock. */
-static void printk_start_kthread(struct console *con)
-{
-	/*
-	 * Do not start a kthread if there is no write() callback. The
-	 * kthreads assume the write() callback exists.
-	 */
-	if (!con->write)
-		return;
-
-	con->thread = kthread_run(printk_kthread_func, con,
-				  "pr/%s%d", con->name, con->index);
-	if (IS_ERR(con->thread)) {
-		con->thread = NULL;
-		con_printk(KERN_ERR, con, "unable to start printing thread\n");
-		__printk_fallback_preferred_direct();
-		return;
-	}
-}
-
 /*
  * Delayed printk version, for scheduler-internal messages:
  */
-#define PRINTK_PENDING_WAKEUP		0x01
-#define PRINTK_PENDING_DIRECT_OUTPUT	0x02
+#define PRINTK_PENDING_WAKEUP	0x01
+#define PRINTK_PENDING_OUTPUT	0x02
 
 static DEFINE_PER_CPU(int, printk_pending);
 
@@ -3899,14 +3447,10 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
 	int pending = this_cpu_xchg(printk_pending, 0);
 
-	if (pending & PRINTK_PENDING_DIRECT_OUTPUT) {
-		printk_prefer_direct_enter();
-
+	if (pending & PRINTK_PENDING_OUTPUT) {
 		/* If trylock fails, someone else is doing the printing */
 		if (console_trylock())
 			console_unlock();
-
-		printk_prefer_direct_exit();
 	}
 
 	if (pending & PRINTK_PENDING_WAKEUP)
@@ -3931,11 +3475,10 @@ static void __wake_up_klogd(int val)
 	 * prepare_to_wait_event(), which is called after ___wait_event() adds
 	 * the waiter but before it has checked the wait condition.
 	 *
-	 * This pairs with devkmsg_read:A, syslog_print:A, and
-	 * printk_kthread_func:A.
+	 * This pairs with devkmsg_read:A and syslog_print:A.
 	 */
 	if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
-	    (val & PRINTK_PENDING_DIRECT_OUTPUT)) {
+	    (val & PRINTK_PENDING_OUTPUT)) {
 		this_cpu_or(printk_pending, val);
 		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
 	}
@@ -3953,17 +3496,7 @@ void defer_console_output(void)
 	 * New messages may have been added directly to the ringbuffer
 	 * using vprintk_store(), so wake any waiters as well.
 	 */
-	int val = PRINTK_PENDING_WAKEUP;
-
-	/*
-	 * Make sure that some context will print the messages when direct
-	 * printing is allowed. This happens in situations when the kthreads
-	 * may not be as reliable or perhaps unusable.
-	 */
-	if (allow_direct_printing())
-		val |= PRINTK_PENDING_DIRECT_OUTPUT;
-
-	__wake_up_klogd(val);
+	__wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
 }
 
 void printk_trigger_flush(void)

kernel/printk/printk_safe.c

@@ -8,9 +8,7 @@
 #include <linux/smp.h>
 #include <linux/cpumask.h>
 #include <linux/printk.h>
-#include <linux/console.h>
 #include <linux/kprobes.h>
-#include <linux/delay.h>
 
 #include "internal.h"
 
@@ -52,33 +50,3 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	return vprintk_default(fmt, args);
 }
 EXPORT_SYMBOL(vprintk);
-
-/**
- * try_block_console_kthreads() - Try to block console kthreads and
- *	make the global console_lock() avaialble
- *
- * @timeout_ms:        The maximum time (in ms) to wait.
- *
- * Prevent console kthreads from starting processing new messages. Wait
- * until the global console_lock() become available.
- *
- * Context: Can be called in any context.
- */
-void try_block_console_kthreads(int timeout_ms)
-{
-	block_console_kthreads = true;
-
-	/* Do not wait when the console lock could not be safely taken. */
-	if (this_cpu_read(printk_context) || in_nmi())
-		return;
-
-	while (timeout_ms > 0) {
-		if (console_trylock()) {
-			console_unlock();
-			return;
-		}
-
-		udelay(1000);
-		timeout_ms -= 1;
-	}
-}

kernel/rcu/tree_stall.h

@@ -647,7 +647,6 @@ static void print_cpu_stall(unsigned long gps)
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
 	 * RCU CPU stall warnings.
 	 */
-	printk_prefer_direct_enter();
 	trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
 	pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
 	raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
@@ -685,7 +684,6 @@ static void print_cpu_stall(unsigned long gps)
 	 */
 	set_tsk_need_resched(current);
 	set_preempt_need_resched();
-	printk_prefer_direct_exit();
 }
 
 static void check_cpu_stall(struct rcu_data *rdp)

kernel/reboot.c

@@ -82,7 +82,6 @@ void kernel_restart_prepare(char *cmd)
 {
 	blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
 	system_state = SYSTEM_RESTART;
-	try_block_console_kthreads(10000);
 	usermodehelper_disable();
 	device_shutdown();
 }
@@ -271,7 +270,6 @@ static void kernel_shutdown_prepare(enum system_states state)
 	blocking_notifier_call_chain(&reboot_notifier_list,
 		(state == SYSTEM_HALT) ? SYS_HALT : SYS_POWER_OFF, NULL);
 	system_state = state;
-	try_block_console_kthreads(10000);
 	usermodehelper_disable();
 	device_shutdown();
 }
@@ -821,11 +819,9 @@ static int __orderly_reboot(void)
 	ret = run_cmd(reboot_cmd);
 
 	if (ret) {
-		printk_prefer_direct_enter();
 		pr_warn("Failed to start orderly reboot: forcing the issue\n");
 		emergency_sync();
 		kernel_restart(NULL);
-		printk_prefer_direct_exit();
 	}
 
 	return ret;
@@ -838,7 +834,6 @@ static int __orderly_poweroff(bool force)
 	ret = run_cmd(poweroff_cmd);
 
 	if (ret && force) {
-		printk_prefer_direct_enter();
 		pr_warn("Failed to start orderly shutdown: forcing the issue\n");
 
 		/*
@@ -848,7 +843,6 @@ static int __orderly_poweroff(bool force)
 		 */
 		emergency_sync();
 		kernel_power_off();
-		printk_prefer_direct_exit();
 	}
 
 	return ret;
@@ -906,8 +900,6 @@ EXPORT_SYMBOL_GPL(orderly_reboot);
  */
 static void hw_failure_emergency_poweroff_func(struct work_struct *work)
 {
-	printk_prefer_direct_enter();
-
 	/*
 	 * We have reached here after the emergency shutdown waiting period has
 	 * expired. This means orderly_poweroff has not been able to shut off
@@ -924,8 +916,6 @@ static void hw_failure_emergency_poweroff_func(struct work_struct *work)
 	 */
 	pr_emerg("Hardware protection shutdown failed. Trying emergency restart\n");
 	emergency_restart();
-
-	printk_prefer_direct_exit();
 }
 
 static DECLARE_DELAYED_WORK(hw_failure_emergency_poweroff_work,
@@ -964,13 +954,11 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced)
 {
 	static atomic_t allow_proceed = ATOMIC_INIT(1);
 
-	printk_prefer_direct_enter();
-
 	pr_emerg("HARDWARE PROTECTION shutdown (%s)\n", reason);
 
 	/* Shutdown should be initiated only once. */
 	if (!atomic_dec_and_test(&allow_proceed))
-		goto out;
+		return;
 
 	/*
 	 * Queue a backup emergency shutdown in the event of
@@ -978,8 +966,6 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced)
 	 */
 	hw_failure_emergency_poweroff(ms_until_forced);
 	orderly_poweroff(true);
-out:
-	printk_prefer_direct_exit();
 }
 EXPORT_SYMBOL_GPL(hw_protection_shutdown);
 

kernel/watchdog.c

@@ -424,8 +424,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		/* Start period for the next softlockup warning. */
 		update_report_ts();
 
-		printk_prefer_direct_enter();
-
 		pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
 			smp_processor_id(), duration,
 			current->comm, task_pid_nr(current));
@@ -444,8 +442,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
 		if (softlockup_panic)
 			panic("softlockup: hung tasks");
-
-		printk_prefer_direct_exit();
 	}
 
 	return HRTIMER_RESTART;

kernel/watchdog_hld.c

@@ -135,8 +135,6 @@ static void watchdog_overflow_callback(struct perf_event *event,
 		if (__this_cpu_read(hard_watchdog_warn) == true)
 			return;
 
-		printk_prefer_direct_enter();
-
 		pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n",
 			 this_cpu);
 		print_modules();
@@ -157,8 +155,6 @@ static void watchdog_overflow_callback(struct perf_event *event,
 		if (hardlockup_panic)
 			nmi_panic(regs, "Hard LOCKUP");
 
-		printk_prefer_direct_exit();
-
 		__this_cpu_write(hard_watchdog_warn, true);
 		return;
 	}