Date:	Fri, 15 May 2015 15:20:20 +0200
From:	Petr Mladek <pmladek@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	akpm@...ux-foundation.org, davem@...emloft.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Kay Sievers <kay@...y.org>
Subject: Re: [PATCH 3/7] printk: implement support for extended console
 drivers

On Thu 2015-05-14 12:06:22, Tejun Heo wrote:
> printk log_buf keeps various metadata for each message including its
> sequence number and timestamp.  The metadata is currently available
> only through /dev/kmsg and stripped out before passed onto console
> drivers.  We want this metadata to be available to console drivers too
> so that console consumers can get full information including the
> metadata and dictionary, which among other things can be used to
> detect whether messages got lost in transit.
> 
> This patch implements support for extended console drivers.  Consoles
> can indicate that they want extended messages by setting the new
> CON_EXTENDED flag and they'll be fed messages formatted the same way
> as /dev/kmsg.
> 
>  "<level>,<sequnum>,<timestamp>,<contflag>;<message text>\n"
> 
> If extended consoles exist, in-kernel fragment assembly is disabled.
> This ensures that all messages emitted to consoles have full metadata
> including sequence number.  The contflag carries enough information to
> reassemble the fragments from the reader side trivially.  Note that
> this only affects /dev/kmsg.  Regular console and /proc/kmsg outputs
> are not affected by this change.
> 
> * Extended message formatting for console drivers is enabled iff there
>   are registered extended consoles.
> 
> * Comment describing /dev/kmsg message format updated to add missing
>   contflag field and help distinguishing variable from verbatim terms.
> 
> v3: Added pr_info() to inform the user to expect more fragments when
>     ext console is enabled.  Also, Documentation/ABI/testing/dev-kmsg
>     is updated accordingly as suggested by Andrew.
> 
> v2: Disable in-kernel cont buffer if extended consoles are present
>     which simplifies things quite a bit.  Hinted by Petr.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

Reviewed-by: Petr Mladek <pmladek@...e.cz>

It is much easier than the previous approach and pretty
straightforward. If there are any bugs in the LOG_CONT flag usage,
it will make them more visible but they should get fixed anyway.

Best Regards,
Petr

> Cc: Kay Sievers <kay@...y.org>
> Cc: Petr Mladek <pmladek@...e.cz>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  Documentation/ABI/testing/dev-kmsg |  9 ++++++
>  include/linux/console.h            |  1 +
>  kernel/printk/printk.c             | 66 +++++++++++++++++++++++++++++++++-----
>  3 files changed, 68 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
> index bb820be..fff817e 100644
> --- a/Documentation/ABI/testing/dev-kmsg
> +++ b/Documentation/ABI/testing/dev-kmsg
> @@ -98,4 +98,13 @@ Description:	The /dev/kmsg character device node provides userspace access
>  		logic is used internally when messages are printed to the
>  		console, /proc/kmsg or the syslog() syscall.
>  
> +		By default, kernel tries to avoid fragments by concatenating
> +		when it can and fragments are rare; however, when extended
> +		console support is enabled, the in-kernel concatenation is
> +		disabled and /dev/kmsg output will contain more fragments. If
> +		the log consumer performs concatenation, the end result
> +		should be the same. In the future, the in-kernel concatenation
> +		may be removed entirely and /dev/kmsg users are recommended to
> +		implement fragment handling.
> +
>  Users:		dmesg(1), userspace kernel log consumers
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 9f50fb4..bd19434 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -115,6 +115,7 @@ static inline int con_debug_leave(void)
>  #define CON_BOOT	(8)
>  #define CON_ANYTIME	(16) /* Safe to call when cpu is offline */
>  #define CON_BRL		(32) /* Used for a braille device */
> +#define CON_EXTENDED	(64) /* Use the extended output format a la /dev/kmsg */
>  
>  struct console {
>  	char	name[16];
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 51ce4f1..ae980dc 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -85,6 +85,18 @@ static struct lockdep_map console_lock_dep_map = {
>  #endif
>  
>  /*
> + * Number of registered extended console drivers.
> + *
> + * If extended consoles are present, in-kernel cont reassembly is disabled
> + * and each fragment is stored as a separate log entry with proper
> + * continuation flag so that every emitted message has full metadata.  This
> + * doesn't change the result for regular consoles or /proc/kmsg.  For
> + * /dev/kmsg, as long as the reader concatenates messages according to
> + * consecutive continuation flags, the end result should be the same too.
> + */
> +static int nr_ext_console_drivers;
> +
> +/*
>   * Helper macros to handle lockdep when locking/unlocking console_sem. We use
>   * macros instead of functions so that _RET_IP_ contains useful information.
>   */
> @@ -195,7 +207,7 @@ static int console_may_schedule;
>   * need to be changed in the future, when the requirements change.
>   *
>   * /dev/kmsg exports the structured data in the following line format:
> - *   "level,sequnum,timestamp;<message text>\n"
> + *   "<level>,<sequnum>,<timestamp>,<contflag>;<message text>\n"
>   *
>   * The optional key/value pairs are attached as continuation lines starting
>   * with a space character and terminated by a newline. All possible
> @@ -1417,7 +1429,9 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
>   * log_buf[start] to log_buf[end - 1].
>   * The console_lock must be held.
>   */
> -static void call_console_drivers(int level, const char *text, size_t len)
> +static void call_console_drivers(int level,
> +				 const char *ext_text, size_t ext_len,
> +				 const char *text, size_t len)
>  {
>  	struct console *con;
>  
> @@ -1438,7 +1452,10 @@ static void call_console_drivers(int level, const char *text, size_t len)
>  		if (!cpu_online(smp_processor_id()) &&
>  		    !(con->flags & CON_ANYTIME))
>  			continue;
> -		con->write(con, text, len);
> +		if (con->flags & CON_EXTENDED)
> +			con->write(con, ext_text, ext_len);
> +		else
> +			con->write(con, text, len);
>  	}
>  }
>  
> @@ -1581,8 +1598,12 @@ static bool cont_add(int facility, int level, const char *text, size_t len)
>  	if (cont.len && cont.flushed)
>  		return false;
>  
> -	if (cont.len + len > sizeof(cont.buf)) {
> -		/* the line gets too long, split it up in separate records */
> +	/*
> +	 * If ext consoles are present, flush and skip in-kernel
> +	 * continuation.  See nr_ext_console_drivers definition.  Also, if
> +	 * the line gets too long, split it up in separate records.
> +	 */
> +	if (nr_ext_console_drivers || cont.len + len > sizeof(cont.buf)) {
>  		cont_flush(LOG_CONT);
>  		return false;
>  	}
> @@ -1917,9 +1938,19 @@ static struct cont {
>  	u8 level;
>  	bool flushed:1;
>  } cont;
> +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 void call_console_drivers(int level, const char *text, size_t len) {}
> +static ssize_t msg_print_ext_header(char *buf, size_t size,
> +				    struct printk_log *msg, u64 seq,
> +				    enum log_flags prev_flags) { 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; }
> +static void call_console_drivers(int level,
> +				 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, enum log_flags prev,
>  			     bool syslog, char *buf, size_t size) { return 0; }
>  static size_t cont_print_text(char *text, size_t size) { return 0; }
> @@ -2172,7 +2203,7 @@ static void console_cont_flush(char *text, size_t size)
>  	len = cont_print_text(text, size);
>  	raw_spin_unlock(&logbuf_lock);
>  	stop_critical_timings();
> -	call_console_drivers(cont.level, text, len);
> +	call_console_drivers(cont.level, NULL, 0, text, len);
>  	start_critical_timings();
>  	local_irq_restore(flags);
>  	return;
> @@ -2196,6 +2227,7 @@ out:
>   */
>  void console_unlock(void)
>  {
> +	static char ext_text[CONSOLE_EXT_LOG_MAX];
>  	static char text[LOG_LINE_MAX + PREFIX_MAX];
>  	static u64 seen_seq;
>  	unsigned long flags;
> @@ -2214,6 +2246,7 @@ void console_unlock(void)
>  again:
>  	for (;;) {
>  		struct printk_log *msg;
> +		size_t ext_len = 0;
>  		size_t len;
>  		int level;
>  
> @@ -2259,13 +2292,22 @@ skip:
>  		level = msg->level;
>  		len += msg_print_text(msg, console_prev, false,
>  				      text + len, sizeof(text) - len);
> +		if (nr_ext_console_drivers) {
> +			ext_len = msg_print_ext_header(ext_text,
> +						sizeof(ext_text),
> +						msg, console_seq, console_prev);
> +			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);
> +		}
>  		console_idx = log_next(console_idx);
>  		console_seq++;
>  		console_prev = msg->flags;
>  		raw_spin_unlock(&logbuf_lock);
>  
>  		stop_critical_timings();	/* don't trace print latency */
> -		call_console_drivers(level, text, len);
> +		call_console_drivers(level, ext_text, ext_len, text, len);
>  		start_critical_timings();
>  		local_irq_restore(flags);
>  	}
> @@ -2521,6 +2563,11 @@ void register_console(struct console *newcon)
>  		newcon->next = console_drivers->next;
>  		console_drivers->next = newcon;
>  	}
> +
> +	if (newcon->flags & CON_EXTENDED)
> +		if (!nr_ext_console_drivers++)
> +			pr_info("printk: continuation disabled due to ext consoles, expect more fragments in /dev/kmsg\n");
> +
>  	if (newcon->flags & CON_PRINTBUFFER) {
>  		/*
>  		 * console_unlock(); will print out the buffered messages
> @@ -2593,6 +2640,9 @@ int unregister_console(struct console *console)
>  		}
>  	}
>  
> +	if (!res && (console->flags & CON_EXTENDED))
> +		nr_ext_console_drivers--;
> +
>  	/*
>  	 * If this isn't the last console and it has CON_CONSDEV set, we
>  	 * need to set it on the next preferred console.
> -- 
> 2.1.0
> 
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