linux-kernel - Re: [PATCH next v2 1/3] printk: inline log_output(),log_store() in vprintk

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Message-ID: <87v9diyhza.fsf@jogness.linutronix.de>
Date:   Thu, 03 Dec 2020 17:31:29 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     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 v2 1/3] printk: inline log_output(),log_store() in vprintk_store()

On 2020-12-03, Petr Mladek <pmladek@...e.com> wrote:
>> +	if (lflags & LOG_CONT) {
>> +		prb_rec_init_wr(&r, text_len);
>> +		if (prb_reserve_in_last(&e, prb, &r, caller_id, LOG_LINE_MAX)) {
>> +			memcpy(&r.text_buf[r.info->text_len], text, text_len);
>> +			r.info->text_len += text_len;
>> +
>> +			if (lflags & LOG_NEWLINE) {
>> +				r.info->flags |= LOG_NEWLINE;
>> +				prb_final_commit(&e);
>> +			} else {
>> +				prb_commit(&e);
>> +			}
>> +
>> +			return text_len;
>> +		}
>> +	}
>> +
>> +	prb_rec_init_wr(&r, text_len);
>
> This is called in both branches. I would do it just once at the
> beginning.

Actually that leads to a crash when prb_reserve_in_last() fails and the
fallback code uses a record that prb_reserve_in_last() has already
touched. The implementation of prb_reserve_in_last() would have to be
changed so that failure guarantees that @r has not been
modified. Currently prb_reserve_in_last() can fail after modifying @r.

>> +	if (!prb_reserve(&e, prb, &r)) {
>> +		/* truncate the message if it is too long for empty buffer */
>> +		truncate_msg(&text_len, &trunc_msg_len);
>> +
>> +		prb_rec_init_wr(&r, text_len + trunc_msg_len);
>> +		if (!prb_reserve(&e, prb, &r))
>> +			return 0;
>> +	}
>> +
>> +	/* fill message */
>> +	memcpy(&r.text_buf[0], text, text_len);
>> +	if (trunc_msg_len)
>> +		memcpy(&r.text_buf[text_len], trunc_msg, trunc_msg_len);
>> +	r.info->text_len = text_len + trunc_msg_len;
>> +	r.info->facility = facility;
>> +	r.info->level = level & 7;
>> +	r.info->flags = lflags & 0x1f;
>> +	r.info->ts_nsec = ts_nsec;
>
> This is the only location where ts_nsec is used. I would remove the
> variable and call:
>
> 	r.info->ts_nsec = local_clock();

My reason for grabbing the clock at the beginning is so that the
timestamp is as close to the printk() call as possible. IMHO it is a
more deterministic timestamp than if it is taken after reservation(s)
and sprint'ing. I prefer to keep it as it is, but will not object if
such a change is necessary for mailine acceptance.

John Ogness