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Date:	Fri, 18 Jul 2014 12:53:56 +0200
From:	Petr Mládek <pmladek@...e.cz>
To:	Alex Elder <elder@...aro.org>
Cc:	akpm@...ux-foundation.org, bp@...e.de, john.stultz@...aro.org,
	jack@...e.cz, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] printk: LOG_CONT and LOG_NEWLINE are opposites

On Thu 2014-07-17 09:09:07, 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.
> 
> 
> 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@...aro.org>
> ---
>  kernel/printk/printk.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ff1c7ad..1ee1862 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)) {
>  		/*

All the above changes make sense and I support them after all.

> @@ -1686,9 +1690,8 @@ asmlinkage int vprintk_emit(int facility, int level,
>  		if (cont_add(facility, level, text, text_len))
>  			printed_len += text_len;
>  		else
> -			printed_len += log_store(facility, level,
> -						 lflags | LOG_CONT, 0,
> -						 dict, dictlen, text, text_len);
> +			printed_len += log_store(facility, level, lflags, 0,
> +						 NULL, 0, text, text_len);
>  	} else {
>  		bool stored = false;
>  

I do not see real benefit in this change. I think that it would cause
more confusion than help. I would suggest to remove it. Then I will
agree with this patch :-)


Best Regards,
Petr
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