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Subject: Re: [PATCH v3 3/3] printk: use the lockless ringbuffer
To:     John Ogness <john.ogness@linutronix.de>,
        Petr Mladek <pmladek@suse.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul McKenney <paulmck@kernel.org>, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org
References: <20200618144919.9806-1-john.ogness@linutronix.de>
 <20200618144919.9806-4-john.ogness@linutronix.de>
From:   lijiang <lijiang@redhat.com>
Message-ID: <ba338fab-9e98-366b-8fd4-05d4daa14f6d@redhat.com>
Date:   Thu, 2 Jul 2020 16:25:44 +0800
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Hi, John Ogness

About the VMCOREINFO part, I made some tests based on the kernel patch
v3, the makedumpfile and crash-utility can work as expected with your
patch(userspace patch), but, unfortunately, the vmcore-dmesg(kexec-tools)
can't correctly read the printk ring buffer information, and get the
following error:

"Missing the log_buf symbol"

The kexec-tools(vmcore-dmesg) should also have a similar patch, just like
in the makedumpfile and crash-utility.

BTW: If you already have a patch for the kexec-tools, would you mind sharing
it? I will make a test for the vmcore-dmesg.

Thanks.
Lianbo

在 2020年06月18日 22:49, John Ogness 写道:
> Replace the existing ringbuffer usage and implementation with
> lockless ringbuffer usage. Even though the new ringbuffer does not
> require locking, all existing locking is left in place. Therefore,
> this change is purely replacing the underlining ringbuffer.
> 
> Changes that exist due to the ringbuffer replacement:
> 
> - The VMCOREINFO has been updated for the new structures.
> 
> - Dictionary data is now stored in a separate data buffer from the
>   human-readable messages. The dictionary data buffer is set to the
>   same size as the message buffer. Therefore, the total required
>   memory for both dictionary and message data is
>   2 * (2 ^ CONFIG_LOG_BUF_SHIFT) for the initial static buffers and
>   2 * log_buf_len (the kernel parameter) for the dynamic buffers.
> 
> - Record meta-data is now stored in a separate array of descriptors.
>   This is an additional 72 * (2 ^ (CONFIG_LOG_BUF_SHIFT - 5)) bytes
>   for the static array and 72 * (log_buf_len >> 5) bytes for the
>   dynamic array.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
>  include/linux/kmsg_dump.h |   2 -
>  kernel/printk/printk.c    | 944 ++++++++++++++++++++------------------
>  2 files changed, 497 insertions(+), 449 deletions(-)
> 
> diff --git a/include/linux/kmsg_dump.h b/include/linux/kmsg_dump.h
> index 3378bcbe585e..c9b0abe5ca91 100644
> --- a/include/linux/kmsg_dump.h
> +++ b/include/linux/kmsg_dump.h
> @@ -45,8 +45,6 @@ struct kmsg_dumper {
>  	bool registered;
>  
>  	/* private state of the kmsg iterator */
> -	u32 cur_idx;
> -	u32 next_idx;
>  	u64 cur_seq;
>  	u64 next_seq;
>  };
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 8c14835be46c..7642ef634956 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -55,6 +55,7 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/printk.h>
>  
> +#include "printk_ringbuffer.h"
>  #include "console_cmdline.h"
>  #include "braille.h"
>  #include "internal.h"
> @@ -294,30 +295,24 @@ enum con_msg_format_flags {
>  static int console_msg_format = MSG_FORMAT_DEFAULT;
>  
>  /*
> - * The printk log buffer consists of a chain of concatenated variable
> - * length records. Every record starts with a record header, containing
> - * the overall length of the record.
> + * The printk log buffer consists of a sequenced collection of records, each
> + * containing variable length message and dictionary text. Every record
> + * also contains its own meta-data (@info).
>   *
> - * The heads to the first and last entry in the buffer, as well as the
> - * sequence numbers of these entries are maintained when messages are
> - * stored.
> + * Every record meta-data carries the timestamp in microseconds, as well as
> + * the standard userspace syslog level and syslog facility. The usual kernel
> + * messages use LOG_KERN; userspace-injected messages always carry a matching
> + * syslog facility, by default LOG_USER. The origin of every message can be
> + * reliably determined that way.
>   *
> - * If the heads indicate available messages, the length in the header
> - * tells the start next message. A length == 0 for the next message
> - * indicates a wrap-around to the beginning of the buffer.
> + * The human readable log message of a record is available in @text, the
> + * length of the message text in @text_len. The stored message is not
> + * terminated.
>   *
> - * Every record carries the monotonic timestamp in microseconds, as well as
> - * the standard userspace syslog level and syslog facility. The usual
> - * kernel messages use LOG_KERN; userspace-injected messages always carry
> - * a matching syslog facility, by default LOG_USER. The origin of every
> - * message can be reliably determined that way.
> - *
> - * The human readable log message directly follows the message header. The
> - * length of the message text is stored in the header, the stored message
> - * is not terminated.
> - *
> - * Optionally, a message can carry a dictionary of properties (key/value pairs),
> - * to provide userspace with a machine-readable message context.
> + * Optionally, a record can carry a dictionary of properties (key/value
> + * pairs), to provide userspace with a machine-readable message context. The
> + * length of the dictionary is available in @dict_len. The dictionary is not
> + * terminated.
>   *
>   * Examples for well-defined, commonly used property names are:
>   *   DEVICE=b12:8               device identifier
> @@ -331,21 +326,19 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
>   * follows directly after a '=' character. Every property is terminated by
>   * a '\0' character. The last property is not terminated.
>   *
> - * Example of a message structure:
> - *   0000  ff 8f 00 00 00 00 00 00      monotonic time in nsec
> - *   0008  34 00                        record is 52 bytes long
> - *   000a        0b 00                  text is 11 bytes long
> - *   000c              1f 00            dictionary is 23 bytes long
> - *   000e                    03 00      LOG_KERN (facility) LOG_ERR (level)
> - *   0010  69 74 27 73 20 61 20 6c      "it's a l"
> - *         69 6e 65                     "ine"
> - *   001b           44 45 56 49 43      "DEVIC"
> - *         45 3d 62 38 3a 32 00 44      "E=b8:2\0D"
> - *         52 49 56 45 52 3d 62 75      "RIVER=bu"
> - *         67                           "g"
> - *   0032     00 00 00                  padding to next message header
> - *
> - * The 'struct printk_log' buffer header must never be directly exported to
> + * Example of record values:
> + *   record.text_buf       = "it's a line" (unterminated)
> + *   record.dict_buf       = "DEVICE=b8:2\0DRIVER=bug" (unterminated)
> + *   record.info.seq       = 56
> + *   record.info.ts_nsec   = 36863
> + *   record.info.text_len  = 11
> + *   record.info.dict_len  = 22
> + *   record.info.facility  = 0 (LOG_KERN)
> + *   record.info.flags     = 0
> + *   record.info.level     = 3 (LOG_ERR)
> + *   record.info.caller_id = 299 (task 299)
> + *
> + * The 'struct printk_info' buffer must never be directly exported to
>   * userspace, it is a kernel-private implementation detail that might
>   * need to be changed in the future, when the requirements change.
>   *
> @@ -365,23 +358,6 @@ enum log_flags {
>  	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
>  };
>  
> -struct printk_log {
> -	u64 ts_nsec;		/* timestamp in nanoseconds */
> -	u16 len;		/* length of entire record */
> -	u16 text_len;		/* length of text buffer */
> -	u16 dict_len;		/* length of dictionary buffer */
> -	u8 facility;		/* syslog facility */
> -	u8 flags:5;		/* internal record flags */
> -	u8 level:3;		/* syslog level */
> -#ifdef CONFIG_PRINTK_CALLER
> -	u32 caller_id;            /* thread id or processor id */
> -#endif
> -}
> -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> -__packed __aligned(4)
> -#endif
> -;
> -
>  /*
>   * The logbuf_lock protects kmsg buffer, indices, counters.  This can be taken
>   * within the scheduler's rq lock. It must be released before calling
> @@ -421,26 +397,16 @@ DEFINE_RAW_SPINLOCK(logbuf_lock);
>  DECLARE_WAIT_QUEUE_HEAD(log_wait);
>  /* the next printk record to read by syslog(READ) or /proc/kmsg */
>  static u64 syslog_seq;
> -static u32 syslog_idx;
>  static size_t syslog_partial;
>  static bool syslog_time;
>  
> -/* index and sequence number of the first record stored in the buffer */
> -static u64 log_first_seq;
> -static u32 log_first_idx;
> -
> -/* index and sequence number of the next record to store in the buffer */
> -static u64 log_next_seq;
> -static u32 log_next_idx;
> -
>  /* the next printk record to write to the console */
>  static u64 console_seq;
> -static u32 console_idx;
>  static u64 exclusive_console_stop_seq;
> +static unsigned long console_dropped;
>  
>  /* the next printk record to read after the last 'clear' command */
>  static u64 clear_seq;
> -static u32 clear_idx;
>  
>  #ifdef CONFIG_PRINTK_CALLER
>  #define PREFIX_MAX		48
> @@ -453,13 +419,28 @@ static u32 clear_idx;
>  #define LOG_FACILITY(v)		((v) >> 3 & 0xff)
>  
>  /* record buffer */
> -#define LOG_ALIGN __alignof__(struct printk_log)
> +#define LOG_ALIGN __alignof__(unsigned long)
>  #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
>  #define LOG_BUF_LEN_MAX (u32)(1 << 31)
>  static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
>  static char *log_buf = __log_buf;
>  static u32 log_buf_len = __LOG_BUF_LEN;
>  
> +/*
> + * Define the average message size. This only affects the number of
> + * descriptors that will be available. Underestimating is better than
> + * overestimating (too many available descriptors is better than not enough).
> + * The dictionary buffer will be the same size as the text buffer.
> + */
> +#define PRB_AVGBITS 5	/* 32 character average length */
> +
> +_DECLARE_PRINTKRB(printk_rb_static, CONFIG_LOG_BUF_SHIFT - PRB_AVGBITS,
> +		  PRB_AVGBITS, PRB_AVGBITS, &__log_buf[0]);
> +
> +static struct printk_ringbuffer printk_rb_dynamic;
> +
> +static struct printk_ringbuffer *prb = &printk_rb_static;
> +
>  /*
>   * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
>   * per_cpu_areas are initialised. This variable is set to true when
> @@ -484,108 +465,6 @@ u32 log_buf_len_get(void)
>  	return log_buf_len;
>  }
>  
> -/* human readable text of the record */
> -static char *log_text(const struct printk_log *msg)
> -{
> -	return (char *)msg + sizeof(struct printk_log);
> -}
> -
> -/* optional key/value pair dictionary attached to the record */
> -static char *log_dict(const struct printk_log *msg)
> -{
> -	return (char *)msg + sizeof(struct printk_log) + msg->text_len;
> -}
> -
> -/* get record by index; idx must point to valid msg */
> -static struct printk_log *log_from_idx(u32 idx)
> -{
> -	struct printk_log *msg = (struct printk_log *)(log_buf + idx);
> -
> -	/*
> -	 * A length == 0 record is the end of buffer marker. Wrap around and
> -	 * read the message at the start of the buffer.
> -	 */
> -	if (!msg->len)
> -		return (struct printk_log *)log_buf;
> -	return msg;
> -}
> -
> -/* get next record; idx must point to valid msg */
> -static u32 log_next(u32 idx)
> -{
> -	struct printk_log *msg = (struct printk_log *)(log_buf + idx);
> -
> -	/* length == 0 indicates the end of the buffer; wrap */
> -	/*
> -	 * A length == 0 record is the end of buffer marker. Wrap around and
> -	 * read the message at the start of the buffer as *this* one, and
> -	 * return the one after that.
> -	 */
> -	if (!msg->len) {
> -		msg = (struct printk_log *)log_buf;
> -		return msg->len;
> -	}
> -	return idx + msg->len;
> -}
> -
> -/*
> - * Check whether there is enough free space for the given message.
> - *
> - * The same values of first_idx and next_idx mean that the buffer
> - * is either empty or full.
> - *
> - * If the buffer is empty, we must respect the position of the indexes.
> - * They cannot be reset to the beginning of the buffer.
> - */
> -static int logbuf_has_space(u32 msg_size, bool empty)
> -{
> -	u32 free;
> -
> -	if (log_next_idx > log_first_idx || empty)
> -		free = max(log_buf_len - log_next_idx, log_first_idx);
> -	else
> -		free = log_first_idx - log_next_idx;
> -
> -	/*
> -	 * We need space also for an empty header that signalizes wrapping
> -	 * of the buffer.
> -	 */
> -	return free >= msg_size + sizeof(struct printk_log);
> -}
> -
> -static int log_make_free_space(u32 msg_size)
> -{
> -	while (log_first_seq < log_next_seq &&
> -	       !logbuf_has_space(msg_size, false)) {
> -		/* drop old messages until we have enough contiguous space */
> -		log_first_idx = log_next(log_first_idx);
> -		log_first_seq++;
> -	}
> -
> -	if (clear_seq < log_first_seq) {
> -		clear_seq = log_first_seq;
> -		clear_idx = log_first_idx;
> -	}
> -
> -	/* sequence numbers are equal, so the log buffer is empty */
> -	if (logbuf_has_space(msg_size, log_first_seq == log_next_seq))
> -		return 0;
> -
> -	return -ENOMEM;
> -}
> -
> -/* compute the message size including the padding bytes */
> -static u32 msg_used_size(u16 text_len, u16 dict_len, u32 *pad_len)
> -{
> -	u32 size;
> -
> -	size = sizeof(struct printk_log) + text_len + dict_len;
> -	*pad_len = (-size) & (LOG_ALIGN - 1);
> -	size += *pad_len;
> -
> -	return size;
> -}
> -
>  /*
>   * Define how much of the log buffer we could take at maximum. The value
>   * must be greater than two. Note that only half of the buffer is available
> @@ -594,22 +473,26 @@ static u32 msg_used_size(u16 text_len, u16 dict_len, u32 *pad_len)
>  #define MAX_LOG_TAKE_PART 4
>  static const char trunc_msg[] = "<truncated>";
>  
> -static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
> -			u16 *dict_len, u32 *pad_len)
> +static void truncate_msg(u16 *text_len, u16 *trunc_msg_len, u16 *dict_len)
>  {
>  	/*
>  	 * The message should not take the whole buffer. Otherwise, it might
>  	 * get removed too soon.
>  	 */
>  	u32 max_text_len = log_buf_len / MAX_LOG_TAKE_PART;
> +
>  	if (*text_len > max_text_len)
>  		*text_len = max_text_len;
> -	/* enable the warning message */
> +
> +	/* enable the warning message (if there is room) */
>  	*trunc_msg_len = strlen(trunc_msg);
> +	if (*text_len >= *trunc_msg_len)
> +		*text_len -= *trunc_msg_len;
> +	else
> +		*trunc_msg_len = 0;
> +
>  	/* disable the "dict" completely */
>  	*dict_len = 0;
> -	/* compute the size again, count also the warning message */
> -	return msg_used_size(*text_len + *trunc_msg_len, 0, pad_len);
>  }
>  
>  /* insert record into the buffer, discard old ones, update heads */
> @@ -618,60 +501,40 @@ static int log_store(u32 caller_id, int facility, int level,
>  		     const char *dict, u16 dict_len,
>  		     const char *text, u16 text_len)
>  {
> -	struct printk_log *msg;
> -	u32 size, pad_len;
> +	struct prb_reserved_entry e;
> +	struct printk_record r;
>  	u16 trunc_msg_len = 0;
>  
> -	/* number of '\0' padding bytes to next message */
> -	size = msg_used_size(text_len, dict_len, &pad_len);
> +	prb_rec_init_wr(&r, text_len, dict_len);
>  
> -	if (log_make_free_space(size)) {
> +	if (!prb_reserve(&e, prb, &r)) {
>  		/* truncate the message if it is too long for empty buffer */
> -		size = truncate_msg(&text_len, &trunc_msg_len,
> -				    &dict_len, &pad_len);
> +		truncate_msg(&text_len, &trunc_msg_len, &dict_len);
> +		prb_rec_init_wr(&r, text_len + trunc_msg_len, dict_len);
>  		/* survive when the log buffer is too small for trunc_msg */
> -		if (log_make_free_space(size))
> +		if (!prb_reserve(&e, prb, &r))
>  			return 0;
>  	}
>  
> -	if (log_next_idx + size + sizeof(struct printk_log) > log_buf_len) {
> -		/*
> -		 * This message + an additional empty header does not fit
> -		 * at the end of the buffer. Add an empty header with len == 0
> -		 * to signify a wrap around.
> -		 */
> -		memset(log_buf + log_next_idx, 0, sizeof(struct printk_log));
> -		log_next_idx = 0;
> -	}
> -
>  	/* fill message */
> -	msg = (struct printk_log *)(log_buf + log_next_idx);
> -	memcpy(log_text(msg), text, text_len);
> -	msg->text_len = text_len;
> -	if (trunc_msg_len) {
> -		memcpy(log_text(msg) + text_len, trunc_msg, trunc_msg_len);
> -		msg->text_len += trunc_msg_len;
> -	}
> -	memcpy(log_dict(msg), dict, dict_len);
> -	msg->dict_len = dict_len;
> -	msg->facility = facility;
> -	msg->level = level & 7;
> -	msg->flags = flags & 0x1f;
> +	memcpy(&r.text_buf[0], text, text_len);
> +	if (trunc_msg_len)
> +		memcpy(&r.text_buf[text_len], trunc_msg, trunc_msg_len);
> +	if (r.dict_buf)
> +		memcpy(&r.dict_buf[0], dict, dict_len);
> +	r.info->facility = facility;
> +	r.info->level = level & 7;
> +	r.info->flags = flags & 0x1f;
>  	if (ts_nsec > 0)
> -		msg->ts_nsec = ts_nsec;
> +		r.info->ts_nsec = ts_nsec;
>  	else
> -		msg->ts_nsec = local_clock();
> -#ifdef CONFIG_PRINTK_CALLER
> -	msg->caller_id = caller_id;
> -#endif
> -	memset(log_dict(msg) + dict_len, 0, pad_len);
> -	msg->len = size;
> +		r.info->ts_nsec = local_clock();
> +	r.info->caller_id = caller_id;
>  
>  	/* insert message */
> -	log_next_idx += msg->len;
> -	log_next_seq++;
> +	prb_commit(&e);
>  
> -	return msg->text_len;
> +	return (text_len + trunc_msg_len);
>  }
>  
>  int dmesg_restrict = IS_ENABLED(CONFIG_SECURITY_DMESG_RESTRICT);
> @@ -723,13 +586,13 @@ static void append_char(char **pp, char *e, char c)
>  		*(*pp)++ = c;
>  }
>  
> -static ssize_t msg_print_ext_header(char *buf, size_t size,
> -				    struct printk_log *msg, u64 seq)
> +static ssize_t info_print_ext_header(char *buf, size_t size,
> +				     struct printk_info *info)
>  {
> -	u64 ts_usec = msg->ts_nsec;
> +	u64 ts_usec = info->ts_nsec;
>  	char caller[20];
>  #ifdef CONFIG_PRINTK_CALLER
> -	u32 id = msg->caller_id;
> +	u32 id = info->caller_id;
>  
>  	snprintf(caller, sizeof(caller), ",caller=%c%u",
>  		 id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
> @@ -740,8 +603,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
>  	do_div(ts_usec, 1000);
>  
>  	return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
> -			 (msg->facility << 3) | msg->level, seq, ts_usec,
> -			 msg->flags & LOG_CONT ? 'c' : '-', caller);
> +			 (info->facility << 3) | info->level, info->seq,
> +			 ts_usec, info->flags & LOG_CONT ? 'c' : '-', caller);
>  }
>  
>  static ssize_t msg_print_ext_body(char *buf, size_t size,
> @@ -795,10 +658,14 @@ static ssize_t msg_print_ext_body(char *buf, size_t size,
>  /* /dev/kmsg - userspace message inject/listen interface */
>  struct devkmsg_user {
>  	u64 seq;
> -	u32 idx;
>  	struct ratelimit_state rs;
>  	struct mutex lock;
>  	char buf[CONSOLE_EXT_LOG_MAX];
> +
> +	struct printk_info info;
> +	char text_buf[CONSOLE_EXT_LOG_MAX];
> +	char dict_buf[CONSOLE_EXT_LOG_MAX];
> +	struct printk_record record;
>  };
>  
>  static __printf(3, 4) __cold
> @@ -881,7 +748,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  			    size_t count, loff_t *ppos)
>  {
>  	struct devkmsg_user *user = file->private_data;
> -	struct printk_log *msg;
> +	struct printk_record *r = &user->record;
>  	size_t len;
>  	ssize_t ret;
>  
> @@ -893,7 +760,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  		return ret;
>  
>  	logbuf_lock_irq();
> -	while (user->seq == log_next_seq) {
> +	if (!prb_read_valid(prb, user->seq, r)) {
>  		if (file->f_flags & O_NONBLOCK) {
>  			ret = -EAGAIN;
>  			logbuf_unlock_irq();
> @@ -902,30 +769,26 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  
>  		logbuf_unlock_irq();
>  		ret = wait_event_interruptible(log_wait,
> -					       user->seq != log_next_seq);
> +					prb_read_valid(prb, user->seq, r));
>  		if (ret)
>  			goto out;
>  		logbuf_lock_irq();
>  	}
>  
> -	if (user->seq < log_first_seq) {
> +	if (user->seq < prb_first_seq(prb)) {
>  		/* our last seen message is gone, return error and reset */
> -		user->idx = log_first_idx;
> -		user->seq = log_first_seq;
> +		user->seq = prb_first_seq(prb);
>  		ret = -EPIPE;
>  		logbuf_unlock_irq();
>  		goto out;
>  	}
>  
> -	msg = log_from_idx(user->idx);
> -	len = msg_print_ext_header(user->buf, sizeof(user->buf),
> -				   msg, user->seq);
> +	len = info_print_ext_header(user->buf, sizeof(user->buf), r->info);
>  	len += msg_print_ext_body(user->buf + len, sizeof(user->buf) - len,
> -				  log_dict(msg), msg->dict_len,
> -				  log_text(msg), msg->text_len);
> +				  &r->dict_buf[0], r->info->dict_len,
> +				  &r->text_buf[0], r->info->text_len);
>  
> -	user->idx = log_next(user->idx);
> -	user->seq++;
> +	user->seq = r->info->seq + 1;
>  	logbuf_unlock_irq();
>  
>  	if (len > count) {
> @@ -957,8 +820,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
>  	switch (whence) {
>  	case SEEK_SET:
>  		/* the first record */
> -		user->idx = log_first_idx;
> -		user->seq = log_first_seq;
> +		user->seq = prb_first_seq(prb);
>  		break;
>  	case SEEK_DATA:
>  		/*
> @@ -966,13 +828,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
>  		 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
>  		 * changes no global state, and does not clear anything.
>  		 */
> -		user->idx = clear_idx;
>  		user->seq = clear_seq;
>  		break;
>  	case SEEK_END:
>  		/* after the last record */
> -		user->idx = log_next_idx;
> -		user->seq = log_next_seq;
> +		user->seq = prb_next_seq(prb);
>  		break;
>  	case SEEK_CUR:
>  		/*
> @@ -1002,9 +862,9 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
>  	poll_wait(file, &log_wait, wait);
>  
>  	logbuf_lock_irq();
> -	if (user->seq < log_next_seq) {
> +	if (prb_read_valid(prb, user->seq, NULL)) {
>  		/* return error when data has vanished underneath us */
> -		if (user->seq < log_first_seq)
> +		if (user->seq < prb_first_seq(prb))
>  			ret = EPOLLIN|EPOLLRDNORM|EPOLLERR|EPOLLPRI;
>  		else
>  			ret = EPOLLIN|EPOLLRDNORM;
> @@ -1039,9 +899,12 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>  
>  	mutex_init(&user->lock);
>  
> +	prb_rec_init_rd(&user->record, &user->info,
> +			&user->text_buf[0], sizeof(user->text_buf),
> +			&user->dict_buf[0], sizeof(user->dict_buf));
> +
>  	logbuf_lock_irq();
> -	user->idx = log_first_idx;
> -	user->seq = log_first_seq;
> +	user->seq = prb_first_seq(prb);
>  	logbuf_unlock_irq();
>  
>  	file->private_data = user;
> @@ -1082,23 +945,52 @@ const struct file_operations kmsg_fops = {
>   */
>  void log_buf_vmcoreinfo_setup(void)
>  {
> -	VMCOREINFO_SYMBOL(log_buf);
> -	VMCOREINFO_SYMBOL(log_buf_len);
> -	VMCOREINFO_SYMBOL(log_first_idx);
> -	VMCOREINFO_SYMBOL(clear_idx);
> -	VMCOREINFO_SYMBOL(log_next_idx);
> +	VMCOREINFO_SYMBOL(prb);
> +	VMCOREINFO_SYMBOL(printk_rb_static);
> +	VMCOREINFO_SYMBOL(clear_seq);
> +
>  	/*
> -	 * Export struct printk_log size and field offsets. User space tools can
> +	 * Export struct size and field offsets. User space tools can
>  	 * parse it and detect any changes to structure down the line.
>  	 */
> -	VMCOREINFO_STRUCT_SIZE(printk_log);
> -	VMCOREINFO_OFFSET(printk_log, ts_nsec);
> -	VMCOREINFO_OFFSET(printk_log, len);
> -	VMCOREINFO_OFFSET(printk_log, text_len);
> -	VMCOREINFO_OFFSET(printk_log, dict_len);
> -#ifdef CONFIG_PRINTK_CALLER
> -	VMCOREINFO_OFFSET(printk_log, caller_id);
> -#endif
> +
> +	VMCOREINFO_STRUCT_SIZE(printk_ringbuffer);
> +	VMCOREINFO_OFFSET(printk_ringbuffer, desc_ring);
> +	VMCOREINFO_OFFSET(printk_ringbuffer, text_data_ring);
> +	VMCOREINFO_OFFSET(printk_ringbuffer, dict_data_ring);
> +	VMCOREINFO_OFFSET(printk_ringbuffer, fail);
> +
> +	VMCOREINFO_STRUCT_SIZE(prb_desc_ring);
> +	VMCOREINFO_OFFSET(prb_desc_ring, count_bits);
> +	VMCOREINFO_OFFSET(prb_desc_ring, descs);
> +	VMCOREINFO_OFFSET(prb_desc_ring, head_id);
> +	VMCOREINFO_OFFSET(prb_desc_ring, tail_id);
> +
> +	VMCOREINFO_STRUCT_SIZE(prb_desc);
> +	VMCOREINFO_OFFSET(prb_desc, info);
> +	VMCOREINFO_OFFSET(prb_desc, state_var);
> +	VMCOREINFO_OFFSET(prb_desc, text_blk_lpos);
> +	VMCOREINFO_OFFSET(prb_desc, dict_blk_lpos);
> +
> +	VMCOREINFO_STRUCT_SIZE(prb_data_blk_lpos);
> +	VMCOREINFO_OFFSET(prb_data_blk_lpos, begin);
> +	VMCOREINFO_OFFSET(prb_data_blk_lpos, next);
> +
> +	VMCOREINFO_STRUCT_SIZE(printk_info);
> +	VMCOREINFO_OFFSET(printk_info, seq);
> +	VMCOREINFO_OFFSET(printk_info, ts_nsec);
> +	VMCOREINFO_OFFSET(printk_info, text_len);
> +	VMCOREINFO_OFFSET(printk_info, dict_len);
> +	VMCOREINFO_OFFSET(printk_info, caller_id);
> +
> +	VMCOREINFO_STRUCT_SIZE(prb_data_ring);
> +	VMCOREINFO_OFFSET(prb_data_ring, size_bits);
> +	VMCOREINFO_OFFSET(prb_data_ring, data);
> +	VMCOREINFO_OFFSET(prb_data_ring, head_lpos);
> +	VMCOREINFO_OFFSET(prb_data_ring, tail_lpos);
> +
> +	VMCOREINFO_SIZE(atomic_long_t);
> +	VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);
>  }
>  #endif
>  
> @@ -1176,11 +1068,46 @@ static void __init set_percpu_data_ready(void)
>  	__printk_percpu_data_ready = true;
>  }
>  
> +static unsigned int __init add_to_rb(struct printk_ringbuffer *rb,
> +				     struct printk_record *r)
> +{
> +	struct prb_reserved_entry e;
> +	struct printk_record dest_r;
> +
> +	prb_rec_init_wr(&dest_r, r->info->text_len, r->info->dict_len);
> +
> +	if (!prb_reserve(&e, rb, &dest_r))
> +		return 0;
> +
> +	memcpy(&dest_r.text_buf[0], &r->text_buf[0], dest_r.text_buf_size);
> +	if (dest_r.dict_buf) {
> +		memcpy(&dest_r.dict_buf[0], &r->dict_buf[0],
> +		       dest_r.dict_buf_size);
> +	}
> +	dest_r.info->facility = r->info->facility;
> +	dest_r.info->level = r->info->level;
> +	dest_r.info->flags = r->info->flags;
> +	dest_r.info->ts_nsec = r->info->ts_nsec;
> +	dest_r.info->caller_id = r->info->caller_id;
> +
> +	prb_commit(&e);
> +
> +	return prb_record_text_space(&e);
> +}
> +
> +static char setup_text_buf[CONSOLE_EXT_LOG_MAX] __initdata;
> +static char setup_dict_buf[CONSOLE_EXT_LOG_MAX] __initdata;
> +
>  void __init setup_log_buf(int early)
>  {
> +	struct prb_desc *new_descs;
> +	struct printk_info info;
> +	struct printk_record r;
>  	unsigned long flags;
> +	char *new_dict_buf;
>  	char *new_log_buf;
>  	unsigned int free;
> +	u64 seq;
>  
>  	/*
>  	 * Some archs call setup_log_buf() multiple times - first is very
> @@ -1201,17 +1128,50 @@ void __init setup_log_buf(int early)
>  
>  	new_log_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);
>  	if (unlikely(!new_log_buf)) {
> -		pr_err("log_buf_len: %lu bytes not available\n",
> +		pr_err("log_buf_len: %lu text bytes not available\n",
>  			new_log_buf_len);
>  		return;
>  	}
>  
> +	new_dict_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);
> +	if (unlikely(!new_dict_buf)) {
> +		/* dictionary failure is allowed */
> +		pr_err("log_buf_len: %lu dict bytes not available\n",
> +			new_log_buf_len);
> +	}
> +
> +	new_descs = memblock_alloc((new_log_buf_len >> PRB_AVGBITS) *
> +				   sizeof(struct prb_desc), LOG_ALIGN);
> +	if (unlikely(!new_descs)) {
> +		pr_err("log_buf_len: %lu desc bytes not available\n",
> +			new_log_buf_len >> PRB_AVGBITS);
> +		if (new_dict_buf)
> +			memblock_free(__pa(new_dict_buf), new_log_buf_len);
> +		memblock_free(__pa(new_log_buf), new_log_buf_len);
> +		return;
> +	}
> +
> +	prb_rec_init_rd(&r, &info,
> +			&setup_text_buf[0], sizeof(setup_text_buf),
> +			&setup_dict_buf[0], sizeof(setup_dict_buf));
> +
>  	logbuf_lock_irqsave(flags);
> +
> +	prb_init(&printk_rb_dynamic,
> +		 new_log_buf, bits_per(new_log_buf_len) - 1,
> +		 new_dict_buf, bits_per(new_log_buf_len) - 1,
> +		 new_descs, (bits_per(new_log_buf_len) - 1) - PRB_AVGBITS);
> +
>  	log_buf_len = new_log_buf_len;
>  	log_buf = new_log_buf;
>  	new_log_buf_len = 0;
> -	free = __LOG_BUF_LEN - log_next_idx;
> -	memcpy(log_buf, __log_buf, __LOG_BUF_LEN);
> +
> +	free = __LOG_BUF_LEN;
> +	prb_for_each_record(0, &printk_rb_static, seq, &r)
> +		free -= add_to_rb(&printk_rb_dynamic, &r);
> +
> +	prb = &printk_rb_dynamic;
> +
>  	logbuf_unlock_irqrestore(flags);
>  
>  	pr_info("log_buf_len: %u bytes\n", log_buf_len);
> @@ -1323,18 +1283,18 @@ static size_t print_caller(u32 id, char *buf)
>  #define print_caller(id, buf) 0
>  #endif
>  
> -static size_t print_prefix(const struct printk_log *msg, bool syslog,
> -			   bool time, char *buf)
> +static size_t info_print_prefix(const struct printk_info  *info, bool syslog,
> +				bool time, char *buf)
>  {
>  	size_t len = 0;
>  
>  	if (syslog)
> -		len = print_syslog((msg->facility << 3) | msg->level, buf);
> +		len = print_syslog((info->facility << 3) | info->level, buf);
>  
>  	if (time)
> -		len += print_time(msg->ts_nsec, buf + len);
> +		len += print_time(info->ts_nsec, buf + len);
>  
> -	len += print_caller(msg->caller_id, buf + len);
> +	len += print_caller(info->caller_id, buf + len);
>  
>  	if (IS_ENABLED(CONFIG_PRINTK_CALLER) || time) {
>  		buf[len++] = ' ';
> @@ -1344,72 +1304,150 @@ static size_t print_prefix(const struct printk_log *msg, bool syslog,
>  	return len;
>  }
>  
> -static size_t msg_print_text(const struct printk_log *msg, bool syslog,
> -			     bool time, char *buf, size_t size)
> +static size_t record_print_text(struct printk_record *r, bool syslog,
> +				bool time)
>  {
> -	const char *text = log_text(msg);
> -	size_t text_size = msg->text_len;
> -	size_t len = 0;
> +	size_t text_len = r->info->text_len;
> +	size_t buf_size = r->text_buf_size;
> +	char *text = r->text_buf;
>  	char prefix[PREFIX_MAX];
> -	const size_t prefix_len = print_prefix(msg, syslog, time, prefix);
> +	bool truncated = false;
> +	size_t prefix_len;
> +	size_t line_len;
> +	size_t len = 0;
> +	char *next;
>  
> -	do {
> -		const char *next = memchr(text, '\n', text_size);
> -		size_t text_len;
> +	prefix_len = info_print_prefix(r->info, syslog, time, prefix);
>  
> +	/*
> +	 * Add the prefix for each line by shifting the rest of the text to
> +	 * make room for each prefix. If the buffer is not large enough for
> +	 * all the prefixes, then drop the trailing text and report the
> +	 * largest length that includes full lines with their prefixes.
> +	 *
> +	 * @text_len: bytes of unprocessed text
> +	 * @line_len: bytes of current line (newline _not_ included)
> +	 * @text:     pointer to beginning of current line
> +	 * @len:      number of bytes processed (size of r->text_buf done)
> +	 */
> +	for (;;) {
> +		next = memchr(text, '\n', text_len);
>  		if (next) {
> -			text_len = next - text;
> -			next++;
> -			text_size -= next - text;
> +			line_len = next - text;
>  		} else {
> -			text_len = text_size;
> +			/*
> +			 * No newline. If the text was previously truncated,
> +			 * assume this line was truncated and do not include
> +			 * it.
> +			 */
> +			if (truncated)
> +				break;
> +			line_len = text_len;
>  		}
>  
> -		if (buf) {
> -			if (prefix_len + text_len + 1 >= size - len)
> +		/*
> +		 * Ensure there is enough buffer available to shift this line
> +		 * (and add a newline at the end).
> +		 */
> +		if (len + prefix_len + line_len + 1 > buf_size)
> +			break;
> +
> +		/*
> +		 * Ensure there is enough buffer available to shift all
> +		 * remaining text (and add a newline at the end). If this
> +		 * test fails, then there must be a newline (i.e.
> +		 * text_len > line_len because the previous test succeeded).
> +		 */
> +		if (len + prefix_len + text_len + 1 > buf_size) {
> +			/*
> +			 * Truncate @text_len so that there is enough space
> +			 * for a prefix. A newline will not be added because
> +			 * the last line of the text is now truncated and
> +			 * will not be included.
> +			 */
> +			text_len = (buf_size - len) - prefix_len;
> +
> +			/*
> +			 * Ensure there is still a newline. Otherwise this
> +			 * line may have been truncated and will not be
> +			 * included.
> +			 */
> +			if (memchr(text, '\n', text_len) == NULL)
>  				break;
>  
> -			memcpy(buf + len, prefix, prefix_len);
> -			len += prefix_len;
> -			memcpy(buf + len, text, text_len);
> -			len += text_len;
> -			buf[len++] = '\n';
> -		} else {
> -			/* SYSLOG_ACTION_* buffer size only calculation */
> -			len += prefix_len + text_len + 1;
> +			/* Note that the last line will not be included. */
> +			truncated = true;
>  		}
>  
> -		text = next;
> -	} while (text);
> +		memmove(text + prefix_len, text, text_len);
> +		memcpy(text, prefix, prefix_len);
> +
> +		/* Advance beyond the newly added prefix and existing line. */
> +		text += prefix_len + line_len;
> +
> +		/* The remaining text has only decreased by the line. */
> +		text_len -= line_len;
> +
> +		len += prefix_len + line_len + 1;
> +
> +		if (text_len) {
> +			/* Advance past the newline. */
> +			text++;
> +			text_len--;
> +		} else {
> +			/* The final line, add a newline. */
> +			*text = '\n';
> +			break;
> +		}
> +	}
>  
>  	return len;
>  }
>  
> +static size_t get_record_text_size(struct printk_info *info,
> +				   unsigned int line_count,
> +				   bool syslog, bool time)
> +{
> +	char prefix[PREFIX_MAX];
> +	size_t prefix_len;
> +
> +	prefix_len = info_print_prefix(info, syslog, time, prefix);
> +
> +	/*
> +	 * Each line will be preceded with a prefix. The intermediate
> +	 * newlines are already within the text, but a final trailing
> +	 * newline will be added.
> +	 */
> +	return ((prefix_len * line_count) + info->text_len + 1);
> +}
> +
>  static int syslog_print(char __user *buf, int size)
>  {
> +	struct printk_info info;
> +	struct printk_record r;
>  	char *text;
> -	struct printk_log *msg;
>  	int len = 0;
>  
>  	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
>  	if (!text)
>  		return -ENOMEM;
>  
> +	prb_rec_init_rd(&r, &info, text, LOG_LINE_MAX + PREFIX_MAX, NULL, 0);
> +
>  	while (size > 0) {
>  		size_t n;
>  		size_t skip;
>  
>  		logbuf_lock_irq();
> -		if (syslog_seq < log_first_seq) {
> -			/* messages are gone, move to first one */
> -			syslog_seq = log_first_seq;
> -			syslog_idx = log_first_idx;
> -			syslog_partial = 0;
> -		}
> -		if (syslog_seq == log_next_seq) {
> +		if (!prb_read_valid(prb, syslog_seq, &r)) {
>  			logbuf_unlock_irq();
>  			break;
>  		}
> +		if (r.info->seq != syslog_seq) {
> +			/* message is gone, move to next valid one */
> +			syslog_seq = r.info->seq;
> +			syslog_partial = 0;
> +		}
>  
>  		/*
>  		 * To keep reading/counting partial line consistent,
> @@ -1419,13 +1457,10 @@ static int syslog_print(char __user *buf, int size)
>  			syslog_time = printk_time;
>  
>  		skip = syslog_partial;
> -		msg = log_from_idx(syslog_idx);
> -		n = msg_print_text(msg, true, syslog_time, text,
> -				   LOG_LINE_MAX + PREFIX_MAX);
> +		n = record_print_text(&r, true, syslog_time);
>  		if (n - syslog_partial <= size) {
>  			/* message fits into buffer, move forward */
> -			syslog_idx = log_next(syslog_idx);
> -			syslog_seq++;
> +			syslog_seq = r.info->seq + 1;
>  			n -= syslog_partial;
>  			syslog_partial = 0;
>  		} else if (!len){
> @@ -1456,55 +1491,49 @@ static int syslog_print(char __user *buf, int size)
>  
>  static int syslog_print_all(char __user *buf, int size, bool clear)
>  {
> +	struct printk_info info;
> +	unsigned int line_count;
> +	struct printk_record r;
>  	char *text;
>  	int len = 0;
> -	u64 next_seq;
>  	u64 seq;
> -	u32 idx;
>  	bool time;
>  
>  	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
>  	if (!text)
>  		return -ENOMEM;
>  
> +	prb_rec_init_rd(&r, &info, text, LOG_LINE_MAX + PREFIX_MAX, NULL, 0);
> +
>  	time = printk_time;
>  	logbuf_lock_irq();
>  	/*
>  	 * Find first record that fits, including all following records,
>  	 * into the user-provided buffer for this dump.
>  	 */
> -	seq = clear_seq;
> -	idx = clear_idx;
> -	while (seq < log_next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> -
> -		len += msg_print_text(msg, true, time, NULL, 0);
> -		idx = log_next(idx);
> -		seq++;
> +	prb_for_each_record(clear_seq, prb, seq, &r) {
> +		line_count = prb_count_lines(text, r.info->text_len);
> +		len += get_record_text_size(r.info, line_count, true, time);
>  	}
>  
>  	/* move first record forward until length fits into the buffer */
> -	seq = clear_seq;
> -	idx = clear_idx;
> -	while (len > size && seq < log_next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> -
> -		len -= msg_print_text(msg, true, time, NULL, 0);
> -		idx = log_next(idx);
> -		seq++;
> +	prb_for_each_record(clear_seq, prb, seq, &r) {
> +		if (len <= size)
> +			break;
> +		line_count = prb_count_lines(text, r.info->text_len);
> +		len -= get_record_text_size(r.info, line_count, true, time);
>  	}
>  
> -	/* last message fitting into this dump */
> -	next_seq = log_next_seq;
> -
>  	len = 0;
> -	while (len >= 0 && seq < next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> -		int textlen = msg_print_text(msg, true, time, text,
> -					     LOG_LINE_MAX + PREFIX_MAX);
> +	prb_for_each_record(seq, prb, seq, &r) {
> +		int textlen;
>  
> -		idx = log_next(idx);
> -		seq++;
> +		textlen = record_print_text(&r, true, time);
> +
> +		if (len + textlen > size) {
> +			seq--;
> +			break;
> +		}
>  
>  		logbuf_unlock_irq();
>  		if (copy_to_user(buf + len, text, textlen))
> @@ -1513,17 +1542,12 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
>  			len += textlen;
>  		logbuf_lock_irq();
>  
> -		if (seq < log_first_seq) {
> -			/* messages are gone, move to next one */
> -			seq = log_first_seq;
> -			idx = log_first_idx;
> -		}
> +		if (len < 0)
> +			break;
>  	}
>  
> -	if (clear) {
> -		clear_seq = log_next_seq;
> -		clear_idx = log_next_idx;
> -	}
> +	if (clear)
> +		clear_seq = seq;
>  	logbuf_unlock_irq();
>  
>  	kfree(text);
> @@ -1533,8 +1557,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
>  static void syslog_clear(void)
>  {
>  	logbuf_lock_irq();
> -	clear_seq = log_next_seq;
> -	clear_idx = log_next_idx;
> +	clear_seq = prb_next_seq(prb);
>  	logbuf_unlock_irq();
>  }
>  
> @@ -1542,6 +1565,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
>  {
>  	bool clear = false;
>  	static int saved_console_loglevel = LOGLEVEL_DEFAULT;
> +	char *text = NULL;
>  	int error;
>  
>  	error = check_syslog_permissions(type, source);
> @@ -1561,7 +1585,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
>  		if (!access_ok(buf, len))
>  			return -EFAULT;
>  		error = wait_event_interruptible(log_wait,
> -						 syslog_seq != log_next_seq);
> +				prb_read_valid(prb, syslog_seq, NULL));
>  		if (error)
>  			return error;
>  		error = syslog_print(buf, len);
> @@ -1609,11 +1633,15 @@ int do_syslog(int type, char __user *buf, int len, int source)
>  		break;
>  	/* Number of chars in the log buffer */
>  	case SYSLOG_ACTION_SIZE_UNREAD:
> +		if (source != SYSLOG_FROM_PROC) {
> +			text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
> +			if (!text)
> +				return -ENOMEM;
> +		}
>  		logbuf_lock_irq();
> -		if (syslog_seq < log_first_seq) {
> +		if (syslog_seq < prb_first_seq(prb)) {
>  			/* messages are gone, move to first one */
> -			syslog_seq = log_first_seq;
> -			syslog_idx = log_first_idx;
> +			syslog_seq = prb_first_seq(prb);
>  			syslog_partial = 0;
>  		}
>  		if (source == SYSLOG_FROM_PROC) {
> @@ -1622,24 +1650,29 @@ int do_syslog(int type, char __user *buf, int len, int source)
>  			 * for pending data, not the size; return the count of
>  			 * records, not the length.
>  			 */
> -			error = log_next_seq - syslog_seq;
> +			error = prb_next_seq(prb) - syslog_seq;
>  		} else {
> -			u64 seq = syslog_seq;
> -			u32 idx = syslog_idx;
>  			bool time = syslog_partial ? syslog_time : printk_time;
> -
> -			while (seq < log_next_seq) {
> -				struct printk_log *msg = log_from_idx(idx);
> -
> -				error += msg_print_text(msg, true, time, NULL,
> -							0);
> +			struct printk_info info;
> +			unsigned int line_count;
> +			struct printk_record r;
> +			u64 seq;
> +
> +			prb_rec_init_rd(&r, &info, text,
> +					LOG_LINE_MAX + PREFIX_MAX, NULL, 0);
> +
> +			prb_for_each_record(syslog_seq, prb, seq, &r) {
> +				line_count = prb_count_lines(text,
> +							r.info->text_len);
> +				error += get_record_text_size(r.info,
> +							      line_count,
> +							      true, time);
>  				time = printk_time;
> -				idx = log_next(idx);
> -				seq++;
>  			}
>  			error -= syslog_partial;
>  		}
>  		logbuf_unlock_irq();
> +		kfree(text);
>  		break;
>  	/* Size of the log buffer */
>  	case SYSLOG_ACTION_SIZE_BUFFER:
> @@ -1806,10 +1839,22 @@ static int console_trylock_spinning(void)
>  static void call_console_drivers(const char *ext_text, size_t ext_len,
>  				 const char *text, size_t len)
>  {
> +	static char dropped_text[64];
> +	size_t dropped_len = 0;
>  	struct console *con;
>  
>  	trace_console_rcuidle(text, len);
>  
> +	if (!console_drivers)
> +		return;
> +
> +	if (console_dropped) {
> +		dropped_len = snprintf(dropped_text, sizeof(dropped_text),
> +				       "** %lu printk messages dropped **\n",
> +				       console_dropped);
> +		console_dropped = 0;
> +	}
> +
>  	for_each_console(con) {
>  		if (exclusive_console && con != exclusive_console)
>  			continue;
> @@ -1822,8 +1867,11 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
>  			continue;
>  		if (con->flags & CON_EXTENDED)
>  			con->write(con, ext_text, ext_len);
> -		else
> +		else {
> +			if (dropped_len)
> +				con->write(con, dropped_text, dropped_len);
>  			con->write(con, text, len);
> +		}
>  	}
>  }
>  
> @@ -1993,7 +2041,6 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	int printed_len;
>  	bool in_sched = false, pending_output;
>  	unsigned long flags;
> -	u64 curr_log_seq;
>  
>  	/* Suppress unimportant messages after panic happens */
>  	if (unlikely(suppress_printk))
> @@ -2009,9 +2056,9 @@ asmlinkage int vprintk_emit(int facility, int level,
>  
>  	/* This stops the holder of console_sem just where we want him */
>  	logbuf_lock_irqsave(flags);
> -	curr_log_seq = log_next_seq;
> +	pending_output = !prb_read_valid(prb, console_seq, NULL);
>  	printed_len = vprintk_store(facility, level, dict, dictlen, fmt, args);
> -	pending_output = (curr_log_seq != log_next_seq);
> +	pending_output &= prb_read_valid(prb, console_seq, NULL);
>  	logbuf_unlock_irqrestore(flags);
>  
>  	/* If called from the scheduler, we can not call up(). */
> @@ -2090,21 +2137,24 @@ EXPORT_SYMBOL(printk);
>  #define PREFIX_MAX		0
>  #define printk_time		false
>  
> +#define prb_read_valid(rb, seq, r)	false
> +#define prb_first_seq(rb)		0
> +
>  static u64 syslog_seq;
> -static u32 syslog_idx;
>  static u64 console_seq;
> -static u32 console_idx;
>  static u64 exclusive_console_stop_seq;
> -static u64 log_first_seq;
> -static u32 log_first_idx;
> -static u64 log_next_seq;
> -static char *log_text(const struct printk_log *msg) { return NULL; }
> -static char *log_dict(const struct printk_log *msg) { return NULL; }
> -static struct printk_log *log_from_idx(u32 idx) { return NULL; }
> -static u32 log_next(u32 idx) { return 0; }
> -static ssize_t msg_print_ext_header(char *buf, size_t size,
> -				    struct printk_log *msg,
> -				    u64 seq) { return 0; }
> +static unsigned long console_dropped;
> +
> +static size_t record_print_text(const struct printk_record *r,
> +				bool syslog, bool time)
> +{
> +	return 0;
> +}
> +static ssize_t info_print_ext_header(char *buf, size_t size,
> +				     struct printk_info *info)
> +{
> +	return 0;
> +}
>  static ssize_t msg_print_ext_body(char *buf, size_t size,
>  				  char *dict, size_t dict_len,
>  				  char *text, size_t text_len) { return 0; }
> @@ -2112,8 +2162,6 @@ static void console_lock_spinning_enable(void) { }
>  static int console_lock_spinning_disable_and_check(void) { return 0; }
>  static void call_console_drivers(const char *ext_text, size_t ext_len,
>  				 const char *text, size_t len) {}
> -static size_t msg_print_text(const struct printk_log *msg, bool syslog,
> -			     bool time, char *buf, size_t size) { return 0; }
>  static bool suppress_message_printing(int level) { return false; }
>  
>  #endif /* CONFIG_PRINTK */
> @@ -2398,14 +2446,19 @@ void console_unlock(void)
>  {
>  	static char ext_text[CONSOLE_EXT_LOG_MAX];
>  	static char text[LOG_LINE_MAX + PREFIX_MAX];
> +	static char dict[LOG_LINE_MAX];
>  	unsigned long flags;
>  	bool do_cond_resched, retry;
> +	struct printk_info info;
> +	struct printk_record r;
>  
>  	if (console_suspended) {
>  		up_console_sem();
>  		return;
>  	}
>  
> +	prb_rec_init_rd(&r, &info, text, sizeof(text), dict, sizeof(dict));
> +
>  	/*
>  	 * Console drivers are called with interrupts disabled, so
>  	 * @console_may_schedule should be cleared before; however, we may
> @@ -2436,35 +2489,26 @@ void console_unlock(void)
>  	}
>  
>  	for (;;) {
> -		struct printk_log *msg;
>  		size_t ext_len = 0;
>  		size_t len;
>  
>  		printk_safe_enter_irqsave(flags);
>  		raw_spin_lock(&logbuf_lock);
> -		if (console_seq < log_first_seq) {
> -			len = snprintf(text, sizeof(text),
> -				       "** %llu printk messages dropped **\n",
> -				       log_first_seq - console_seq);
> -
> -			/* messages are gone, move to first one */
> -			console_seq = log_first_seq;
> -			console_idx = log_first_idx;
> -		} else {
> -			len = 0;
> -		}
>  skip:
> -		if (console_seq == log_next_seq)
> +		if (!prb_read_valid(prb, console_seq, &r))
>  			break;
>  
> -		msg = log_from_idx(console_idx);
> -		if (suppress_message_printing(msg->level)) {
> +		if (console_seq != r.info->seq) {
> +			console_dropped += r.info->seq - console_seq;
> +			console_seq = r.info->seq;
> +		}
> +
> +		if (suppress_message_printing(r.info->level)) {
>  			/*
>  			 * Skip record we have buffered and already printed
>  			 * directly to the console when we received it, and
>  			 * record that has level above the console loglevel.
>  			 */
> -			console_idx = log_next(console_idx);
>  			console_seq++;
>  			goto skip;
>  		}
> @@ -2475,19 +2519,24 @@ void console_unlock(void)
>  			exclusive_console = NULL;
>  		}
>  
> -		len += msg_print_text(msg,
> -				console_msg_format & MSG_FORMAT_SYSLOG,
> -				printk_time, text + len, sizeof(text) - len);
> +		/*
> +		 * Handle extended console text first because later
> +		 * record_print_text() will modify the record buffer in-place.
> +		 */
>  		if (nr_ext_console_drivers) {
> -			ext_len = msg_print_ext_header(ext_text,
> +			ext_len = info_print_ext_header(ext_text,
>  						sizeof(ext_text),
> -						msg, console_seq);
> +						r.info);
>  			ext_len += msg_print_ext_body(ext_text + ext_len,
>  						sizeof(ext_text) - ext_len,
> -						log_dict(msg), msg->dict_len,
> -						log_text(msg), msg->text_len);
> +						&r.dict_buf[0],
> +						r.info->dict_len,
> +						&r.text_buf[0],
> +						r.info->text_len);
>  		}
> -		console_idx = log_next(console_idx);
> +		len = record_print_text(&r,
> +				console_msg_format & MSG_FORMAT_SYSLOG,
> +				printk_time);
>  		console_seq++;
>  		raw_spin_unlock(&logbuf_lock);
>  
> @@ -2527,7 +2576,7 @@ void console_unlock(void)
>  	 * flush, no worries.
>  	 */
>  	raw_spin_lock(&logbuf_lock);
> -	retry = console_seq != log_next_seq;
> +	retry = prb_read_valid(prb, console_seq, NULL);
>  	raw_spin_unlock(&logbuf_lock);
>  	printk_safe_exit_irqrestore(flags);
>  
> @@ -2596,8 +2645,7 @@ void console_flush_on_panic(enum con_flush_mode mode)
>  		unsigned long flags;
>  
>  		logbuf_lock_irqsave(flags);
> -		console_seq = log_first_seq;
> -		console_idx = log_first_idx;
> +		console_seq = prb_first_seq(prb);
>  		logbuf_unlock_irqrestore(flags);
>  	}
>  	console_unlock();
> @@ -2840,7 +2888,6 @@ void register_console(struct console *newcon)
>  		exclusive_console = newcon;
>  		exclusive_console_stop_seq = console_seq;
>  		console_seq = syslog_seq;
> -		console_idx = syslog_idx;
>  		logbuf_unlock_irqrestore(flags);
>  	}
>  	console_unlock();
> @@ -3229,9 +3276,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
>  
>  		logbuf_lock_irqsave(flags);
>  		dumper->cur_seq = clear_seq;
> -		dumper->cur_idx = clear_idx;
> -		dumper->next_seq = log_next_seq;
> -		dumper->next_idx = log_next_idx;
> +		dumper->next_seq = prb_next_seq(prb);
>  		logbuf_unlock_irqrestore(flags);
>  
>  		/* invoke dumper which will iterate over records */
> @@ -3265,28 +3310,33 @@ void kmsg_dump(enum kmsg_dump_reason reason)
>  bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
>  			       char *line, size_t size, size_t *len)
>  {
> -	struct printk_log *msg;
> +	struct printk_info info;
> +	unsigned int line_count;
> +	struct printk_record r;
>  	size_t l = 0;
>  	bool ret = false;
>  
> +	prb_rec_init_rd(&r, &info, line, size, NULL, 0);
> +
>  	if (!dumper->active)
>  		goto out;
>  
> -	if (dumper->cur_seq < log_first_seq) {
> -		/* messages are gone, move to first available one */
> -		dumper->cur_seq = log_first_seq;
> -		dumper->cur_idx = log_first_idx;
> -	}
> -
> -	/* last entry */
> -	if (dumper->cur_seq >= log_next_seq)
> -		goto out;
> +	/* Read text or count text lines? */
> +	if (line) {
> +		if (!prb_read_valid(prb, dumper->cur_seq, &r))
> +			goto out;
> +		l = record_print_text(&r, syslog, printk_time);
> +	} else {
> +		if (!prb_read_valid_info(prb, dumper->cur_seq,
> +					 &info, &line_count)) {
> +			goto out;
> +		}
> +		l = get_record_text_size(&info, line_count, syslog,
> +					 printk_time);
>  
> -	msg = log_from_idx(dumper->cur_idx);
> -	l = msg_print_text(msg, syslog, printk_time, line, size);
> +	}
>  
> -	dumper->cur_idx = log_next(dumper->cur_idx);
> -	dumper->cur_seq++;
> +	dumper->cur_seq = r.info->seq + 1;
>  	ret = true;
>  out:
>  	if (len)
> @@ -3347,23 +3397,25 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
>  bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
>  			  char *buf, size_t size, size_t *len)
>  {
> +	struct printk_info info;
> +	unsigned int line_count;
> +	struct printk_record r;
>  	unsigned long flags;
>  	u64 seq;
> -	u32 idx;
>  	u64 next_seq;
> -	u32 next_idx;
>  	size_t l = 0;
>  	bool ret = false;
>  	bool time = printk_time;
>  
> -	if (!dumper->active)
> +	prb_rec_init_rd(&r, &info, buf, size, NULL, 0);
> +
> +	if (!dumper->active || !buf || !size)
>  		goto out;
>  
>  	logbuf_lock_irqsave(flags);
> -	if (dumper->cur_seq < log_first_seq) {
> +	if (dumper->cur_seq < prb_first_seq(prb)) {
>  		/* messages are gone, move to first available one */
> -		dumper->cur_seq = log_first_seq;
> -		dumper->cur_idx = log_first_idx;
> +		dumper->cur_seq = prb_first_seq(prb);
>  	}
>  
>  	/* last entry */
> @@ -3374,41 +3426,41 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
>  
>  	/* calculate length of entire buffer */
>  	seq = dumper->cur_seq;
> -	idx = dumper->cur_idx;
> -	while (seq < dumper->next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> -
> -		l += msg_print_text(msg, true, time, NULL, 0);
> -		idx = log_next(idx);
> -		seq++;
> +	while (prb_read_valid_info(prb, seq, &info, &line_count)) {
> +		if (r.info->seq >= dumper->next_seq)
> +			break;
> +		l += get_record_text_size(&info, line_count, true, time);
> +		seq = r.info->seq + 1;
>  	}
>  
>  	/* move first record forward until length fits into the buffer */
>  	seq = dumper->cur_seq;
> -	idx = dumper->cur_idx;
> -	while (l >= size && seq < dumper->next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> -
> -		l -= msg_print_text(msg, true, time, NULL, 0);
> -		idx = log_next(idx);
> -		seq++;
> +	while (l >= size && prb_read_valid_info(prb, seq,
> +						&info, &line_count)) {
> +		if (r.info->seq >= dumper->next_seq)
> +			break;
> +		l -= get_record_text_size(&info, line_count, true, time);
> +		seq = r.info->seq + 1;
>  	}
>  
>  	/* last message in next interation */
>  	next_seq = seq;
> -	next_idx = idx;
>  
> +	/* actually read text into the buffer now */
>  	l = 0;
> -	while (seq < dumper->next_seq) {
> -		struct printk_log *msg = log_from_idx(idx);
> +	while (prb_read_valid(prb, seq, &r)) {
> +		if (r.info->seq >= dumper->next_seq)
> +			break;
> +
> +		l += record_print_text(&r, syslog, time);
> +
> +		/* adjust record to store to remaining buffer space */
> +		prb_rec_init_rd(&r, &info, buf + l, size - l, NULL, 0);
>  
> -		l += msg_print_text(msg, syslog, time, buf + l, size - l);
> -		idx = log_next(idx);
> -		seq++;
> +		seq = r.info->seq + 1;
>  	}
>  
>  	dumper->next_seq = next_seq;
> -	dumper->next_idx = next_idx;
>  	ret = true;
>  	logbuf_unlock_irqrestore(flags);
>  out:
> @@ -3431,9 +3483,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
>  void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
>  {
>  	dumper->cur_seq = clear_seq;
> -	dumper->cur_idx = clear_idx;
> -	dumper->next_seq = log_next_seq;
> -	dumper->next_idx = log_next_idx;
> +	dumper->next_seq = prb_next_seq(prb);
>  }
>  
>  /**
>