linux-kernel - Re: [PATCHv7 4/8] printk: always use deferred printk when flush printk

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Date:   Mon, 6 Feb 2017 11:55:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 4/8] printk: always use deferred printk when flush
 printk_safe lines

On Tue, 7 Feb 2017 01:42:53 +0900
Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> On (02/06/17 08:27), Steven Rostedt wrote:
> [..]
> > > > > just in case, the patch (which I prefer to be ignored)    
> > > > 
> > > > let's keep printk_safe_flush_line().    
> > > 
> > > I do not have strong opinion but I would slightly prefer
> > > to keep the helper function. The use of printk_deferred()
> > > is a bit tricky and it is better to have only one copy.
> > > 
> > > Steven, could you live with the original patch, please?  
> > 
> > Sure, but make it into a static inline.  
> 
> well, I'll be surprised if gcc doesn't inline that simple one-liner.

I never trust gcc ;-)  It's been known to uninline simple one liners
before that have been marked as "inline" :-p

> 
> attached. no conflicts, the patch can replace 0004.
> 
> ===8<===8<===
> 
> >From 69bbb0f436a2a89ec41a5831c03490e0a78ce12e Mon Sep 17 00:00:00 2001  
> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Date: Tue, 27 Dec 2016 23:16:07 +0900
> Subject: [PATCH] printk: always use deferred printk when flush printk_safe
>  lines
> 
> Always use printk_deferred() in printk_safe_flush_line().
> Flushing can be done from NMI or printk_safe contexts (when
> we are in panic), so we can't call console drivers, yet still
> want to store the messages in the logbuf buffer. Therefore we
> use a deferred printk version.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

> ---
>  kernel/printk/printk_safe.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> index efc89a4e9df5..5214d326d3ba 100644
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -110,17 +110,15 @@ static int printk_safe_log_store(struct printk_safe_seq_buf *s,
>  	return add;
>  }
>  
> -static void printk_safe_flush_line(const char *text, int len)
> +static inline void printk_safe_flush_line(const char *text, int len)
>  {
>  	/*
> -	 * The buffers are flushed in NMI only on panic.  The messages must
> -	 * go only into the ring buffer at this stage.  Consoles will get
> -	 * explicitly called later when a crashdump is not generated.
> +	 * Avoid any console drivers calls from here, because we may be
> +	 * in NMI or printk_safe context (when in panic). The messages
> +	 * must go only into the ring buffer at this stage.  Consoles will
> +	 * get explicitly called later when a crashdump is not generated.
>  	 */
> -	if (in_nmi())
> -		printk_deferred("%.*s", len, text);
> -	else
> -		printk("%.*s", len, text);
> +	printk_deferred("%.*s", len, text);
>  }
>  
>  /* printk part of the temporary buffer line by line */