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

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Message-ID: <20190626224703.GL2490@worktop.programming.kicks-ass.net>
Date:   Thu, 27 Jun 2019 00:47:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        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 Fri, Jun 21, 2019 at 12:23:19AM +0200, John Ogness wrote:
> On 2019-06-18, Peter Zijlstra <peterz@...radead.org> wrote:
> >> +
> >> +	if (unlikely(newest_id == EOL)) {
> >> +		/* no previous newest means we *are* the list, set oldest */
> >> +
> >> +		/*
> >> +		 * MB UNPAIRED
> >
> > That's a bug, MB must always be paired.
> 
> Well, it "pairs" with the smp_rmb() of the readers, but I didn't think
> that counts as a pair. That's why I wrote unpaired. The litmus test is:
> 
> P0(int *x, int *y)
> {
>         WRITE_ONCE(*x, 1);
>         smp_store_release(y, 1);
> }
> 
> P1(int *x, int *y)
> {
>         int rx;
>         int ry;
> 
>         ry = READ_ONCE(*y);
>         smp_rmb();
>         rx = READ_ONCE(*x);
> }
> 
> exists (1:rx=0 /\ 1:ry=1)
> 
> The readers rely on the store_releases "pairing" with the smp_rmb() so
> that the readers see things in a sane order.

That is certainly a valid pairing, see also the 'SMP BARRIER PAIRING'
section in memory-barriers.txt (I thought we had a table in there, but
apparently that never quite made it in).