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Date:   Wed, 20 Feb 2019 13:18:25 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: Pass caller information to log_store().

On Sat 2019-02-16 19:59:33, Tetsuo Handa wrote:
> When thread1 called printk() which did not end with '\n', and then thread2
> called printk() which ends with '\n' before thread1 calls pr_cont(), the
> partial content saved into "struct cont" is flushed by thread2 despite the
> partial content was generated by thread1. This leads to confusing output
> as if the partial content was generated by thread2. Fix this problem by
> passing correct caller information to log_store().
> 
> Before:
> 
>   [ T8533] abcdefghijklm
>   [ T8533] ABCDEFGHIJKLMNOPQRSTUVWXYZ
>   [ T8532] nopqrstuvwxyz
>   [ T8532] abcdefghijklmnopqrstuvwxyz
>   [ T8533] abcdefghijklm
>   [ T8533] ABCDEFGHIJKLMNOPQRSTUVWXYZ
>   [ T8532] nopqrstuvwxyz
> 
> After:
> 
>   [ T8507] abcdefghijklm
>   [ T8508] ABCDEFGHIJKLMNOPQRSTUVWXYZ
>   [ T8507] nopqrstuvwxyz
>   [ T8507] abcdefghijklmnopqrstuvwxyz
>   [ T8507] abcdefghijklm
>   [ T8508] ABCDEFGHIJKLMNOPQRSTUVWXYZ
>   [ T8507] nopqrstuvwxyz

Great catch!
 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

The patch looks fine to me:

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

There are just two cosmetic changes that I might do when pushing
the patch, see below.

> ---
>  kernel/printk/printk.c | 39 ++++++++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 33c0359..6547a88 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1850,13 +1854,16 @@ static bool cont_add(int facility, int level, enum log_flags flags, const char *
>  
>  static size_t log_output(int facility, int level, enum log_flags lflags, const char *dict, size_t dictlen, char *text, size_t text_len)
>  {
> +	const u32 caller_id = printk_caller_id();
> +
>  	/*
>  	 * If an earlier line was buffered, and we're a continuation
> -	 * write from the same process, try to add it to the buffer.
> +	 * write from the same context, try to add it to the buffer.
>  	 */
>  	if (cont.len) {
> -		if (cont.owner == current && (lflags & LOG_CONT)) {
> -			if (cont_add(facility, level, lflags, text, text_len))
> +		if (cont.caller_id == caller_id && (lflags & LOG_CONT)) {
> +			if (cont_add(caller_id, facility, level, lflags, text,
> +				     text_len))

I think that this is the case when it is better to break the 80-character
per-line rule.

>  				return text_len;
>  		}
>  		/* Otherwise, make sure it's flushed */
> @@ -1869,12 +1876,14 @@ static size_t log_output(int facility, int level, enum log_flags lflags, const c
>  
>  	/* If it doesn't end in a newline, try to buffer the current line */
>  	if (!(lflags & LOG_NEWLINE)) {
> -		if (cont_add(facility, level, lflags, text, text_len))
> +		if (cont_add(caller_id, facility, level, lflags, text,
> +			     text_len))

Same here.

>  			return text_len;
>  	}
>  

Best Regards,
Petr

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