Merge tag 'printk-for-5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Make %pK behave the same as %p for kptr_restrict == 0 also with
   no_hash_pointers parameter

 - Ignore the default console in the device tree also when console=null
   or console="" is used on the command line

 - Document console=null and console="" behavior

 - Prevent a deadlock and a livelock caused by console_lock in panic()

 - Make console_lock available for panicking CPU

 - Fast query for the next to-be-used sequence number

 - Use the expected return values in printk.devkmsg __setup handler

 - Use the correct atomic operations in wake_up_klogd() irq_work handler

 - Avoid possible unaligned access when handling %4cc printing format

* tag 'printk-for-5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk: fix return value of printk.devkmsg __setup handler
  vsprintf: Fix %pK with kptr_restrict == 0
  printk: make suppress_panic_printk static
  printk: Set console_set_on_cmdline=1 when __add_preferred_console() is called with user_specified == true
  Docs: printk: add 'console=null|""' to admin/kernel-parameters
  printk: use atomic updates for klogd work
  printk: Drop console_sem during panic
  printk: Avoid livelock with heavy printk during panic
  printk: disable optimistic spin during panic
  printk: Add panic_in_progress helper
  vsprintf: Move space out of string literals in fourcc_string()
  vsprintf: Fix potential unaligned access
  printk: ringbuffer: Improve prb_next_seq() performance

Author: torvalds

Documentation/admin-guide/kernel-parameters.txt

@@ -724,6 +724,12 @@
 		hvc<n>	Use the hypervisor console device <n>. This is for
 			both Xen and PowerPC hypervisors.
 
+		{ null | "" }
+			Use to disable console output, i.e., to have kernel
+			console messages discarded.
+			This must be the only console= parameter used on the
+			kernel command line.
+
 		If the device connected to the port is not a TTY but a braille
 		device, prepend "brl," before the device type, for instance
 			console=brl,ttyS0
@@ -3516,8 +3522,7 @@
 			difficult since unequal pointers can no longer be
 			compared.  However, if this command-line option is
 			specified, then all normal pointers will have their true
-			value printed.  Pointers printed via %pK may still be
-			hashed.  This option should only be specified when
+			value printed. This option should only be specified when
 			debugging the kernel.  Please do not use on production
 			kernels.
 

kernel/printk/printk.c

@@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers);
  */
 int __read_mostly suppress_printk;
 
+/*
+ * During panic, heavy printk by other CPUs can delay the
+ * panic and risk deadlock on console resources.
+ */
+static int __read_mostly suppress_panic_printk;
+
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {
 	.name = "console_lock"
@@ -146,8 +152,10 @@ static int __control_devkmsg(char *str)
 
 static int __init control_devkmsg(char *str)
 {
-	if (__control_devkmsg(str) < 0)
+	if (__control_devkmsg(str) < 0) {
+		pr_warn("printk.devkmsg: bad option string '%s'\n", str);
 		return 1;
+	}
 
 	/*
 	 * Set sysctl string accordingly:
@@ -166,7 +174,7 @@ static int __init control_devkmsg(char *str)
 	 */
 	devkmsg_log |= DEVKMSG_LOG_MASK_LOCK;
 
-	return 0;
+	return 1;
 }
 __setup("printk.devkmsg=", control_devkmsg);
 
@@ -257,6 +265,11 @@ static void __up_console_sem(unsigned long ip)
 }
 #define up_console_sem() __up_console_sem(_RET_IP_)
 
+static bool panic_in_progress(void)
+{
+	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
+}
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's
@@ -1843,6 +1856,16 @@ static int console_trylock_spinning(void)
 	if (console_trylock())
 		return 1;
 
+	/*
+	 * It's unsafe to spin once a panic has begun. If we are the
+	 * panic CPU, we may have already halted the owner of the
+	 * console_sem. If we are not the panic CPU, then we should
+	 * avoid taking console_sem, so the panic CPU has a better
+	 * chance of cleanly acquiring it later.
+	 */
+	if (panic_in_progress())
+		return 0;
+
 	printk_safe_enter_irqsave(flags);
 
 	raw_spin_lock(&console_owner_lock);
@@ -2218,6 +2241,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (unlikely(suppress_printk))
 		return 0;
 
+	if (unlikely(suppress_panic_printk) &&
+	    atomic_read(&panic_cpu) != raw_smp_processor_id())
+		return 0;
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -2324,6 +2351,20 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
 }
 #endif
 
+static void set_user_specified(struct console_cmdline *c, bool user_specified)
+{
+	if (!user_specified)
+		return;
+
+	/*
+	 * @c console was defined by the user on the command line.
+	 * Do not clear when added twice also by SPCR or the device tree.
+	 */
+	c->user_specified = true;
+	/* At least one console defined by the user on the command line. */
+	console_set_on_cmdline = 1;
+}
+
 static int __add_preferred_console(char *name, int idx, char *options,
 				   char *brl_options, bool user_specified)
 {
@@ -2340,8 +2381,7 @@ static int __add_preferred_console(char *name, int idx, char *options,
 		if (strcmp(c->name, name) == 0 && c->index == idx) {
 			if (!brl_options)
 				preferred_console = i;
-			if (user_specified)
-				c->user_specified = true;
+			set_user_specified(c, user_specified);
 			return 0;
 		}
 	}
@@ -2351,7 +2391,7 @@ static int __add_preferred_console(char *name, int idx, char *options,
 		preferred_console = i;
 	strlcpy(c->name, name, sizeof(c->name));
 	c->options = options;
-	c->user_specified = user_specified;
+	set_user_specified(c, user_specified);
 	braille_set_options(c, brl_options);
 
 	c->index = idx;
@@ -2417,7 +2457,6 @@ static int __init console_setup(char *str)
 	*s = 0;
 
 	__add_preferred_console(buf, idx, options, brl_options, true);
-	console_set_on_cmdline = 1;
 	return 1;
 }
 __setup("console=", console_setup);
@@ -2573,6 +2612,25 @@ static int have_callable_console(void)
 	return 0;
 }
 
+/*
+ * Return true when this CPU should unlock console_sem without pushing all
+ * messages to the console. This reduces the chance that the console is
+ * locked when the panic CPU tries to use it.
+ */
+static bool abandon_console_lock_in_panic(void)
+{
+	if (!panic_in_progress())
+		return false;
+
+	/*
+	 * We can use raw_smp_processor_id() here because it is impossible for
+	 * the task to be migrated to the panic_cpu, or away from it. If
+	 * panic_cpu has already been set, and we're not currently executing on
+	 * that CPU, then we never will be.
+	 */
+	return atomic_read(&panic_cpu) != raw_smp_processor_id();
+}
+
 /*
  * Can we actually use the console at this time on this cpu?
  *
@@ -2603,6 +2661,7 @@ void console_unlock(void)
 {
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[CONSOLE_LOG_MAX];
+	static int panic_console_dropped;
 	unsigned long flags;
 	bool do_cond_resched, retry;
 	struct printk_info info;
@@ -2657,6 +2716,10 @@ void console_unlock(void)
 		if (console_seq != r.info->seq) {
 			console_dropped += r.info->seq - console_seq;
 			console_seq = r.info->seq;
+			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");
+			}
 		}
 
 		if (suppress_message_printing(r.info->level)) {
@@ -2716,6 +2779,10 @@ void console_unlock(void)
 		if (handover)
 			return;
 
+		/* Allow panic_cpu to take over the consoles safely */
+		if (abandon_console_lock_in_panic())
+			break;
+
 		if (do_cond_resched)
 			cond_resched();
 	}
@@ -2733,7 +2800,7 @@ void console_unlock(void)
 	 * flush, no worries.
 	 */
 	retry = prb_read_valid(prb, next_seq, NULL);
-	if (retry && console_trylock())
+	if (retry && !abandon_console_lock_in_panic() && console_trylock())
 		goto again;
 }
 EXPORT_SYMBOL(console_unlock);
@@ -3228,7 +3295,7 @@ static DEFINE_PER_CPU(int, printk_pending);
 
 static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
-	int pending = __this_cpu_xchg(printk_pending, 0);
+	int pending = this_cpu_xchg(printk_pending, 0);
 
 	if (pending & PRINTK_PENDING_OUTPUT) {
 		/* If trylock fails, someone else is doing the printing */
@@ -3262,7 +3329,7 @@ void defer_console_output(void)
 		return;
 
 	preempt_disable();
-	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+	this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
 	preempt_enable();
 }

kernel/printk/printk_ringbuffer.c

@@ -474,8 +474,10 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	 * state has been re-checked. A memcpy() for all of @desc
 	 * cannot be used because of the atomic_t @state_var field.
 	 */
-	memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
-	       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
+	if (desc_out) {
+		memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
+		       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
+	}
 	if (seq_out)
 		*seq_out = info->seq; /* also part of desc_read:C */
 	if (caller_id_out)
@@ -528,7 +530,8 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
 	d_state = get_desc_state(id, state_val);
 out:
-	atomic_long_set(&desc_out->state_var, state_val);
+	if (desc_out)
+		atomic_long_set(&desc_out->state_var, state_val);
 	return d_state;
 }
 
@@ -1449,6 +1452,9 @@ static void desc_make_final(struct prb_desc_ring *desc_ring, unsigned long id)
 
 	atomic_long_cmpxchg_relaxed(&d->state_var, prev_state_val,
 			DESC_SV(id, desc_finalized)); /* LMM(desc_make_final:A) */
+
+	/* Best effort to remember the last finalized @id. */
+	atomic_long_set(&desc_ring->last_finalized_id, id);
 }
 
 /**
@@ -1657,7 +1663,12 @@ void prb_commit(struct prb_reserved_entry *e)
  */
 void prb_final_commit(struct prb_reserved_entry *e)
 {
+	struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
+
 	_prb_commit(e, desc_finalized);
+
+	/* Best effort to remember the last finalized @id. */
+	atomic_long_set(&desc_ring->last_finalized_id, e->id);
 }
 
 /*
@@ -2005,9 +2016,39 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
  */
 u64 prb_next_seq(struct printk_ringbuffer *rb)
 {
-	u64 seq = 0;
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	enum desc_state d_state;
+	unsigned long id;
+	u64 seq;
+
+	/* Check if the cached @id still points to a valid @seq. */
+	id = atomic_long_read(&desc_ring->last_finalized_id);
+	d_state = desc_read(desc_ring, id, NULL, &seq, NULL);
 
-	/* Search forward from the oldest descriptor. */
+	if (d_state == desc_finalized || d_state == desc_reusable) {
+		/*
+		 * Begin searching after the last finalized record.
+		 *
+		 * On 0, the search must begin at 0 because of hack#2
+		 * of the bootstrapping phase it is not known if a
+		 * record at index 0 exists.
+		 */
+		if (seq != 0)
+			seq++;
+	} else {
+		/*
+		 * The information about the last finalized sequence number
+		 * has gone. It should happen only when there is a flood of
+		 * new messages and the ringbuffer is rapidly recycled.
+		 * Give up and start from the beginning.
+		 */
+		seq = 0;
+	}
+
+	/*
+	 * The information about the last finalized @seq might be inaccurate.
+	 * Search forward to find the current one.
+	 */
 	while (_prb_read_valid(rb, &seq, NULL, NULL))
 		seq++;
 
@@ -2044,6 +2085,7 @@ void prb_init(struct printk_ringbuffer *rb,
 	rb->desc_ring.infos = infos;
 	atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
 	atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
+	atomic_long_set(&rb->desc_ring.last_finalized_id, DESC0_ID(descbits));
 
 	rb->text_data_ring.size_bits = textbits;
 	rb->text_data_ring.data = text_buf;

kernel/printk/printk_ringbuffer.h

@@ -75,6 +75,7 @@ struct prb_desc_ring {
 	struct printk_info	*infos;
 	atomic_long_t		head_id;
 	atomic_long_t		tail_id;
+	atomic_long_t		last_finalized_id;
 };
 
 /*
@@ -258,6 +259,7 @@ static struct printk_ringbuffer name = {							\
 		.infos		= &_##name##_infos[0],						\
 		.head_id	= ATOMIC_INIT(DESC0_ID(descbits)),				\
 		.tail_id	= ATOMIC_INIT(DESC0_ID(descbits)),				\
+		.last_finalized_id = ATOMIC_INIT(DESC0_ID(descbits)),				\
 	},											\
 	.text_data_ring = {									\
 		.size_bits	= (avgtextbits) + (descbits),					\