linux-kernel - syslog size unread: was: [PATCH v3 3/3] printk: use the lockless ringbuffer

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Message-ID: <20200625152042.GI8444@alley>
Date:   Thu, 25 Jun 2020 17:20:42 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: syslog size unread: was: [PATCH v3 3/3] printk: use the lockless
 ringbuffer

On Thu 2020-06-18 16:55:19, John Ogness wrote:
> Replace the existing ringbuffer usage and implementation with
> lockless ringbuffer usage. Even though the new ringbuffer does not
> require locking, all existing locking is left in place. Therefore,
> this change is purely replacing the underlining ringbuffer.
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1609,11 +1633,15 @@ int do_syslog(int type, char __user *buf, int len, int source)
>  		break;
>  	/* Number of chars in the log buffer */
>  	case SYSLOG_ACTION_SIZE_UNREAD:
> +		if (source != SYSLOG_FROM_PROC) {
> +			text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
> +			if (!text)
> +				return -ENOMEM;

The buffer is needed only to count lines to count the size of
added prefixes. Could we use the new prb_read_valid_info() that
allows to get the number of lines without actually reading the buffer?

> +
> +		}
>  		logbuf_lock_irq();
> -		if (syslog_seq < log_first_seq) {
> +		if (syslog_seq < prb_first_seq(prb)) {
>  			/* messages are gone, move to first one */
> -			syslog_seq = log_first_seq;
> -			syslog_idx = log_first_idx;
> +			syslog_seq = prb_first_seq(prb);
>  			syslog_partial = 0;
>  		}
>  		if (source == SYSLOG_FROM_PROC) {

Best Regards,
Petr