linux-kernel - Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi

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Date:   Mon, 29 Aug 2016 17:16:00 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] printk/nmi: avoid direct printk()-s from
 __printk_nmi_flush()

On Mon 2016-08-29 21:32:20, Sergey Senozhatsky wrote:
> From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
> 
> __printk_nmi_flush() can be called from nmi_panic(), therefore it has to
> test whether it's executed in NMI context and thus must route the messages
> through deferred printk() or via direct printk(). Except for two places
> where __printk_nmi_flush() does unconditional direct printk() calls:
>  - pr_err("printk_nmi_flush: internal error ...")
>  - pr_cont("\n")
> 
> Factor out __print_nmi_seq_line(), which takes care of in_nmi(), and use
> it in __printk_nmi_flush() for printing and error-reporting.

Great catch! The check and eventually printk_deferred() need to be used
everywhere in __printk_nmi_flush().

Just some nitpicks below.

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  kernel/printk/nmi.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
> index b69eb8a..5f2c198 100644
> --- a/kernel/printk/nmi.c
> +++ b/kernel/printk/nmi.c
> @@ -103,23 +103,32 @@ again:
>   * printk one line from the temporary buffer from @start index until
>   * and including the @end index.
>   */

The comment above is not longer valid.

> -static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
> +static void __print_nmi_seq_line(const char *text, int len)

Also the name of the function might be confusing because it is not
longer used only for the seq buffer. I would rename it to
something like:

       printk_nmi_flush_line()

>  {
> -	const char *buf = s->buffer + start;
> -
>  	/*
>  	 * 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.
>  	 */
>  	if (in_nmi())
> -		printk_deferred("%.*s", (end - start) + 1, buf);
> +		printk_deferred("%.*s", len, text);
>  	else
> -		printk("%.*s", (end - start) + 1, buf);
> +		printk("%.*s", len, text);
>  
>  }
>  
>  /*
> + * printk one line from the temporary buffer from @start index until
> + * and including the @end index.
> + */
> +static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
> +{

Then I would rename also this function to something like:

     printk_nmi_flush_seq_line()

> +	const char *buf = s->buffer + start;
> +
> +	__print_nmi_seq_line(buf, (end - start) + 1);
> +}
> +

Othrewise, it looks fine. With the above suggested changes, feel
free to add:

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

Best Regards,
Petr