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

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Message-ID: <87pnn0yapl.fsf@linutronix.de>
Date:   Wed, 26 Jun 2019 11:09:26 +0200
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>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

On 2019-06-26, Petr Mladek <pmladek@...e.com> wrote:
>> To address your question: For the linked list implementation, if you
>> are looking at it from the linked list perspective, the number of
>> descriptors on the list is constantly fluctuating (increasing and
>> decreasing) and the ordering of the descriptors is constantly
>> changing. They are ordered according to the writer commit order (not
>> the writer reserve order) and the only descriptors on the list are
>> the ones that are not within a reserve/commit window.
>
> This and few other comments below are really valuable explanation.
> I misunderstood how the list worked.

I will add a documentation section about why a linked list was used.

>>>>> If the above is true then we could achieve similar result
>>>>> when using the array as a circular buffer. It would be
>>>>> the same like when all members are linked from the beginning.
>>>> 
>>>> So you are suggesting using a multi-reader multi-writer lockless
>>>> ringbuffer to implement a multi-reader multi-writer lockless
>>>> ringbuffer. ;-)
>>>> 
>>>> The descriptor ringbuffer has fixed-size items, which simplifies
>>>> the task. But I expect you will run into a chicken-egg scenario.
>>>
>>> AFAIK, the main obstacle with the fully lockless solution was
>>> that the entries did not have a fixed size.
>> 
>> No. The variable size of the records was the reason I used
>> descriptors. That has nothing to do with how I chose to connect those
>> descriptors.
>
> I think that we are talking about the same. If I remember correctly,
> the main problem is that cmpxchg() is not reliable when the same
> address might be used by the metadata and data.

The cmpxchg() issue you mention is why I needed descriptors. But even if
I were to implement a fixed-record-size ringbuffer where the cmpxchg()
issue does not exist, I _still_ would have used a linked list to connect
the records.

It is misleading to think the linked list is because of variable size
records.

John Ogness