linux-kernel - Re: [PATCH v3] printk: Have printk() never buffer its data

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Date:	Wed, 27 Jun 2012 22:00:14 -0700
From:	Joe Perches <joe@...ches.com>
To:	Kay Sievers <kay@...y.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v3] printk: Have printk() never buffer its data

On Thu, 2012-06-28 at 09:38 +0200, Kay Sievers wrote:
> Here we share the continuation buffer with the console copy logic,
> and partial lines are always immediately flushed to the available
> consoles. They are still buffered internally to improve the
> readability and integrity of the messages and minimize the amount
> of needed record headers to store.

trivia:

> diff --git a/kernel/printk.c b/kernel/printk.c

> @@ -329,8 +337,13 @@ static void log_store(int facility, int level,
>  	msg->text_len = text_len;
>  	memcpy(log_dict(msg), dict, dict_len);
>  	msg->dict_len = dict_len;
> -	msg->level = (facility << 3) | (level & 7);
> -	msg->ts_nsec = local_clock();
> +	msg->facility = facility;
> +	msg->level = level & 7;

Doesn't need & 7

> +	msg->flags = flags & 0x1f;
> +	if (ts_nsec > 0)
> +		msg->ts_nsec = ts_nsec;
> +	else
> +		msg->ts_nsec = local_clock();

Why local_clock?  ts_nsec really can be 0.
[]

> @@ -1294,15 +1311,92 @@ static inline void printk_delay(void)
>  	}
>  }
>  
> +/*
> + * Continuation lines are buffered, and not committed to the record buffer
> + * until the line is complete, or a race forces it. The line fragments
> + * though, are printed immediately to the consoles to ensure everything has
> + * reached the console in case of a kernel crash.
> + */
> +static struct cont {
> +	char buf[LOG_LINE_MAX];
> +	size_t len;			/* length == 0 means unused buffer */
> +	size_t cons;			/* bytes written to console */
> +	struct task_struct *owner;	/* task of first print*/
> +	u64 ts_nsec;			/* time of first print */
> +	u8 level;			/* log level of first message */
> +	u8 facility;			/* log level of first message */
> +	bool flushed:1;			/* buffer sealed and committed */

bool flushed:1 seems unnecessary.
bool flushed seems more appropriate.

> +} cont;
> +
> +static void cont_flush(void)
> +{
> +	if (cont.flushed)
> +		return;
> +	if (cont.len == 0)
> +		return;
> +
> +	log_store(cont.facility, cont.level, LOG_NOCONS, cont.ts_nsec,
> +		  NULL, 0, cont.buf, cont.len);
> +
> +	cont.flushed = true;
> +}
> +
> +static bool cont_add(int facility, int level, const char *text, size_t len)
> +{
> +	if (cont.len && cont.flushed)
> +		return false;
> +
> +	if (cont.len + len > sizeof(cont.buf)) {
> +		cont_flush();
> +		return false;
> +	}
> +
> +	if (!cont.len) {
> +		cont.facility = facility;
> +		cont.level = level;
> +		cont.owner = current;
> +		cont.ts_nsec = local_clock();
> +		cont.cons = 0;
> +		cont.flushed = false;
> +	}
> +
> +	memcpy(cont.buf + cont.len, text, len);

Looks like you can still overrun cont.buf.


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