linux-kernel - Re: [PATCH next v3 3/3] printk: remove logbuf

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Date:   Wed, 9 Dec 2020 05:35:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v3 3/3] printk: remove logbuf_lock protection for
 ringbuffer

On (20/12/07 23:26), John Ogness wrote:
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e1f068677a74..f3c0fcc3163f 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1068,7 +1068,6 @@ void __init setup_log_buf(int early)
>  	struct printk_record r;
>  	size_t new_descs_size;
>  	size_t new_infos_size;
> -	unsigned long flags;
>  	char *new_log_buf;
>  	unsigned int free;
>  	u64 seq;
> @@ -1126,8 +1125,6 @@ void __init setup_log_buf(int early)
>  		 new_descs, ilog2(new_descs_count),
>  		 new_infos);
>  
> -	logbuf_lock_irqsave(flags);
> -
>  	log_buf_len = new_log_buf_len;
>  	log_buf = new_log_buf;
>  	new_log_buf_len = 0;
> @@ -1143,8 +1140,6 @@ void __init setup_log_buf(int early)
>  	 */
>  	prb = &printk_rb_dynamic;
>  
> -	logbuf_unlock_irqrestore(flags);

logbuf_lock_irqsave() does two things - it locks the logbuf lock and
enables the printk_safe gating. While we can drop the former, the
latter one must stay until we have a complete replacement.

Looking more:

> ---
>	logbuf_lock_irqsave(flags);
>
>	log_buf_len = new_log_buf_len;
>	log_buf = new_log_buf;
>	new_log_buf_len = 0;
>
>	free = __LOG_BUF_LEN;
>	prb_for_each_record(0, &printk_rb_static, seq, &r)
>		free -= add_to_rb(&printk_rb_dynamic, &r);
>
>	/*
>	 * This is early enough that everything is still running on the
>	 * boot CPU and interrupts are disabled. So no new messages will
>	 * appear during the transition to the dynamic buffer.
>	 */
>	prb = &printk_rb_dynamic;
>
>	logbuf_unlock_irqrestore(flags);
---

I'd say that I'd prefer to keep logbuf initialisation under printk_safe(),
per-CPU buffer can be already initialised at this point. Strictly speaking
we can have new message during transition to the dynamic buffer - there are
functions there that can pr_err/warn while we prb_for_each_record/add_to_rb.

>	-ss