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

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Message-ID: <20190626211610.GY7905@worktop.programming.kicks-ass.net>
Date:   Wed, 26 Jun 2019 23:16:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.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 Mon, Jun 24, 2019 at 10:33:15AM +0200, John Ogness wrote:
> Here are the writer-relevant memory barriers and their associated
> variables:
> 
> MB1: data_list.oldest
> MB2: data_list.newest
> MB3: data_block.id
> MB4: descr.data_next
> MB5: descr_list.newest
> MB6: descr.next

I think this is the fundamental divergence in parlance.

You seem to associate a barrier with a (single) variable, where normally
a barrier is between two (or more) variables.

As you wrote in that other email (I'm stlil going through all that);
your MB5 isn't desc_list.newest, but rather between desc_list.newest and
descr.next.

Remember, the topic is called 'memory ordering', and you cannot order
singles.