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

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Date:   Wed, 26 Jun 2019 23:43:56 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 2019-06-26, Peter Zijlstra <peterz@...radead.org> 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.

The litmus tests I posted to answer your previous questions should
(hopefully) show that I already understand this. The above list shows
the _key_ loads/stores that are used to guarantee ordering (for these
and other memory operations). And yes, I now understand that my comments
need to list all the operations that are being ordered based on these
key loads/stores.

> 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.

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.

John Ogness