linux-kernel - Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

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Message-ID: <20190618045117.GA7419@jagdpanzerIV>
Date:   Tue, 18 Jun 2019 13:51:17 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer
 implementation

Hello John,

On (06/07/19 18:29), John Ogness wrote:
[..]
> +	struct prb_reserved_entry e;
> +	char *s;
> +
> +	s = prb_reserve(&e, &rb, 32);
> +	if (s) {
> +		sprintf(s, "Hello, world!");
> +		prb_commit(&e);
> +	}

A nit: snprintf().

sprintf() is tricky, it may write "slightly more than was
anticipated" bytes - all those string_nocheck(" disabled"),
error_string("pK-error"), etc.

[..]
> +Sample reader code::
> +
> +	DECLARE_PRINTKRB_ENTRY(entry, 128);
> +	DECLARE_PRINTKRB_ITER(iter, &test_rb, &entry);
> +	u64 last_seq = 0;
> +	int len;
> +	char *s;
> +
> +	prb_for_each_entry(&iter, len) {
> +		if (entry.seq - last_seq != 1) {
> +			printf("LOST %llu ENTRIES\n",
> +				entry.seq - (last_seq + 1));
> +		}
> +		last_seq = entry.seq;
> +
> +		s = (char *)&entry.buffer[0];
> +		if (len >= 128)
> +			s[128 - 1] = 0;
> +		printf("data: %s\n", s);
> +	}

How are we going to handle pr_cont() loops?

print_modules()
 preempt_disable();
 list_for_each_entry_rcu(mod, &modules, list) {
  pr_cont(" %s%s", mod->name, module_flags(mod, buf));
 }
 preempt_enable();

	-ss