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

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Message-ID: <20190627082803.7aduwbwxwejyhgan@pathway.suse.cz>
Date:   Thu, 27 Jun 2019 10:28:03 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Wed 2019-06-26 23:43:56, John Ogness wrote:
> Here is where I have massive problems communicating. I don't understand
> why you say the barrier is _between_ newest and next. I would say the
> barrier is _on_ newest to _synchronize_ with next (or something). I am
> struggling with terminology. (To be honest, I'd much rather just post
> litmus tests.)
> 
> For example, if we have:
> 
> WRITE_ONCE(&a, 1);
> WRITE_ONCE(&b, 1);
> WRITE_ONCE(&c, 1);
> smp_store_release(&d, 1);
> 
> and:
> 
> local_d = smp_load_acquire(&d);
> local_a = READ_ONCE(&a);
> local_b = READ_ONCE(&b);
> local_c = READ_ONCE(&c);
> 
> How do you describe that? Do you say the memory barrier is between a and
> d? Or between a, b, c, d? (a, b, c aren't ordered, but they are one-way
> synchronized with d).
> 
> I would say there is a barrier on d to synchronize a, b, c.

Barriers are always paired. We need to know what variables are
synchonized against each other, what is the reason and where
is the counter part.

I think that it might be done many ways. I am familiar with
bariers in kernel/livepatch/ code. They use rather long description.
But I find it pretty useful especially when the problem is
complicated and more bariers are involved in a single
transition.

Best Regards,
Petr