linux-kernel - Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)

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Date:   Tue, 03 Dec 2019 15:13:36 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        kexec@...ts.infradead.org
Subject: Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)

On 2019-12-02, Petr Mladek <pmladek@...e.com> wrote:
>> +/*
>> + * Sanity checker for reserve size. The ringbuffer code assumes that a data
>> + * block does not exceed the maximum possible size that could fit within the
>> + * ringbuffer. This function provides that basic size check so that the
>> + * assumption is safe.
>> + */
>> +static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
>> +{
>> +	struct prb_data_block *db = NULL;
>> +
>> +	/* Writers are not allowed to write data-less records. */
>> +	if (size == 0)
>> +		return false;
>
> I would personally let this decision to the API caller.
>
> The code actually have to support data-less records. They are used
> when the descriptor is reserved but the data block can't get reserved.

Exactly. Data-less records are how the ringbuffer identifies that data
has been lost. If users were allowed to store data-less records, that
destinction is no longer possible (unless I created some extra field in
the descriptor). Does it even make sense for printk to store data-less
records explicitly?

The current printk implementation silently ignores empty messages.

> The above statement might create false feeling that it could not
> happen later in the code. It might lead to bugs in writer code.

Then let me change the statement to describe that data-less records are
used internally by the ringbuffer and cannot be explicitly created by
writers.

> Also reader API users might not expect to get a "valid" data-less
> record.

Readers will never see them. The reader API implementation skips over
data-less records.

John Ogness