[v3,3/7] printk: LOG_CONT and LOG_NEWLINE are opposites

Two log record flags--LOG_CONT and LOG_NEWLINE--are mutually
exclusive.  That is, one or the other is always set, but they are
never both set at the same time in a log record flags field.  What
follows is a great deal of explanation that aims to prove this
assertion.

Having that knowledge allows us to simplify a bit of logic, and with
a little more work (in follow-on patches) it allows us to do without
some flag values, considerably simplifying things.


Log record flags are held in the "cont" continuation buffer, as well
as in "syslog_prev" and "console_prev".  Those are discussed later.

Other than that, log record flags are only set in log_store():
    msg->flags = flags & 0x1f;
There are 5 places log_store() is called:  twice from cont_flush();
and three times from vprintk_emit().

Only two single-flag values are ever passed to cont_flush():
LOG_CONT; and LOG_NEWLINE.  That passed-in value is provided to
log_store() either as-is, or modified to include LOG_NOCONS.  There
are thus four possible flag combinations supplied to log_store() by
cont_flush():  LOG_CONT; LOG_NEWLINE; LOG_CONT|LOG_NOCONS; or
LOG_NEWLINE|LOG_NOCONS.

The first call vprintk_emit() makes to log_store() passes a flags
value of LOG_PREFIX|LOG_NEWLINE.  The second and third calls pass a
locally-computed "lflags" value, possibly with LOG_CONT added.  The
only possible flag combinations held in "lflags" are:  0;
LOG_NEWLINE; LOG_PREFIX; or LOG_NEWLINE|LOG_PREFIX.  If LOG_NEWLINE
is not set, LOG_CONT flag is added in the call to log_store().  So
there are four possible flag combinations supplied by cont_flush():
LOG_CONT; LOG_NEWLINE; LOG_PREFIX|LOG_CONT; or LOG_PREFIX|LOG_NEWLINE.

Therefore log_store() is never provided (so never records) a flag
value that contains both LOG_CONT and LOG_NEWLINE, and one of those
flags is always present.


Meanwhile, the "cont" flags field is only ever assigned value 0, or
a value passed to cont_flush().  The value of cont.flags is only
ever *used* if cont.flushed is true, and that only gets set after
the cont.flags field is assigned to the value passed to
cont_flush().  As mentioned above, cont_flush() is never provided
more than one flag value, and it's always either LOG_NEWLINE
or LOG_CONT.  Therefore, for all intents and purposes, cont.flags
only ever holds LOG_NEWLINE or LOG_CONT.

The only values assigned to "syslog_prev" and "console_prev" are the
initial value LOG_NEWLINE or the flags value from a log record.  So
none of these ever hold LOG_CONT and LOG_NEWLINE at the same time.

This proves that at LOG_CONT and LOG_NEWLINE are in fact mutually
exclusive flags.

This patch makes the following changes:
    - Changes two spots in msg_print_txt() so they no longer bother
      checking for LOG_NEWLINE once its known that LOG_CONT is set.
    - Changes two |= assignments in vprintk_emit() to be simple
      assignments instead, because the result is known to be the
      same and it makes it obvious no other flags are involved.
    - Assigns LOG_CONT to "lflags" in vprintk_emit() if it does not
      contain LOG_NEWLINE (rather than just adding it at the time
      of the call to log_store()).
    - Adds a short explanatory comment.
    - Explicitly pass NULL rather than "dict" (and 0 as its length)
      to log_store() in one spot in vprintk_emit(), because we know
      the value of dict at that point is always NULL.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Petr Mládek <pmladek@suse.cz>
---
 kernel/printk/printk.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

On 07/18/2014 09:14 AM, Alex Elder wrote:
> Two log record flags--LOG_CONT and LOG_NEWLINE--are mutually
> exclusive.  That is, one or the other is always set, but they are
> never both set at the same time in a log record flags field.  What
> follows is a great deal of explanation that aims to prove this
> assertion.
> 
> Having that knowledge allows us to simplify a bit of logic, and with
> a little more work (in follow-on patches) it allows us to do without
> some flag values, considerably simplifying things.

And in this patch I inadvertently deleted a hunk.  It
goes at the end, and I've appended it below.

I'm really sorry I screwed these patches up.  I got
sloppy, and tried to send them out in a hurry this
morning.

I'm going to post a new version 4 that will correct these
things.

					-Alex

> Signed-off-by: Alex Elder <elder@linaro.org>
> Reviewed-by: Petr Mládek <pmladek@suse.cz>
> ---
>  kernel/printk/printk.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f2a069e..00367b5 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1006,11 +1006,9 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
>  		prefix = false;
>  
>  	if (msg->flags & LOG_CONT) {
> -		if ((prev & LOG_CONT) && !(prev & LOG_NEWLINE))
> +		if (prev & LOG_CONT)
>  			prefix = false;
> -
> -		if (!(msg->flags & LOG_NEWLINE))
> -			newline = false;
> +		newline = false;
>  	}
>  
>  	do {
> @@ -1639,10 +1637,16 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	text_len += vscnprintf(text + text_len,
>  			       sizeof(textbuf) - text_len, fmt, args);
>  
> -	/* mark and strip a trailing newline */
> +	/*
> +	 * If there's a trailing newline, flag it and strip it off.
> +	 * Otherwise we assume this is a partial log message, to be
> +	 * continued with the next call.
> +	 */
>  	if (text_len && text[text_len-1] == '\n') {
>  		text_len--;
> -		lflags |= LOG_NEWLINE;
> +		lflags = LOG_NEWLINE;
> +	} else {
> +		lflags = LOG_CONT;
>  	}
>  
>  	/* strip kernel syslog prefix and extract log level or control flags */
> @@ -1672,7 +1676,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>  		level = default_message_loglevel;
>  
>  	if (dict)
> -		lflags |= LOG_PREFIX|LOG_NEWLINE;
> +		lflags = LOG_PREFIX|LOG_NEWLINE;
>  
>  	if (!(lflags & LOG_NEWLINE)) {
>  		/*
> 
@@ -1700,8 +1700,7 @@ asmlinkage int vprintk_emit(int facility
                if (cont_add(facility, level, text, text_len))
                        printed_len += text_len;
                else
-                       printed_len += log_store(facility, level,
-                                                lflags | LOG_CONT, 0,
+                       printed_len += log_store(facility, level, lflags, 0,
                                                 dict, dictlen, text,
text_len);
        } else {
                bool stored = false;

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