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

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Date:   Wed, 19 Jun 2019 12:46:55 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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

> I would appreciate it if you could point out a source file that
> documents its memory barriers the way you would like to see these memory
> barriers documented.

IMO, you could find some inspiration by looking at the memory barriers
comments from:

  kernel/sched/core.c:try_to_wake_up()
  include/linux/wait.h:waitqueue_active()
  kernel/futex.c [header _and inline annotations]

I'll detail a single example here, and then conclude with some general
guidelines:

---
[from kernel/sched/rt.c]

static inline void rt_set_overload(struct rq *rq)
{
	if (!rq->online)
		return;

	cpumask_set_cpu(rq->cpu, rq->rd->rto_mask);
	/*
	 * Make sure the mask is visible before we set
	 * the overload count. That is checked to determine
	 * if we should look at the mask. It would be a shame
	 * if we looked at the mask, but the mask was not
	 * updated yet.
	 *
	 * Matched by the barrier in pull_rt_task().
	 */
	smp_wmb();
	atomic_inc(&rq->rd->rto_count);
}

static void pull_rt_task(struct rq *this_rq)
{
	int this_cpu = this_rq->cpu, cpu;
	bool resched = false;
	struct task_struct *p;
	struct rq *src_rq;
	int rt_overload_count = rt_overloaded(this_rq);

	if (likely(!rt_overload_count))
		return;

	/*
	 * Match the barrier from rt_set_overloaded; this guarantees that if we
	 * see overloaded we must also see the rto_mask bit.
	 */
	smp_rmb();

	/* If we are the only overloaded CPU do nothing */
	if (rt_overload_count == 1 &&
	    cpumask_test_cpu(this_rq->cpu, this_rq->rd->rto_mask))
		return;

	[...]

}
---

Notice that the comments provide the following information: for _each_
memory barrier primitive,

  1) the _memory accesses_ being ordered

     the store to ->rto_mask and the store to ->rto_count for the smp_wmb()
     the load from ->rto_count and the from ->rto_mask for the smp_rmb()

  2) the _matching barrier_ (and its location)

  3) an informal description of the _underlying guarantee(s)_  (c.f.,
     "if we see overloaded we must also see the rto_mask bit").

One can provide this information by embedding some snippet/pseudo-code
in its comments as illustrated in the examples pointed out above.

I'd suggest to _not be stingy with memory barriers explanations:  this
eases/makes it possible the review itself as well as future changes or
fixes to the implementation.

FWIW (and as anticipated time ago in a private email), when I see code
like this I tend to look elsewhere...  ;-/

Thanks,
  Andrea