From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation
Date: Wed, 19 Jun 2019 12:46:55 +0200 [thread overview]
Message-ID: <20190619104655.GA6668@andrea> (raw)
In-Reply-To: <87a7eebk71.fsf@linutronix.de>
> 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
next prev parent reply other threads:[~2019-06-19 10:47 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-07 16:23 [RFC PATCH v2 0/2] printk: new ringbuffer implementation John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 1/2] printk-rb: add a new printk " John Ogness
2019-06-18 4:51 ` Sergey Senozhatsky
2019-06-18 22:12 ` John Ogness
2019-06-25 6:45 ` Sergey Senozhatsky
2019-06-25 7:15 ` Sergey Senozhatsky
2019-06-25 8:44 ` John Ogness
2019-06-25 9:06 ` Petr Mladek
2019-06-25 10:03 ` Sergey Senozhatsky
2019-06-25 12:03 ` John Ogness
2019-06-26 2:08 ` Sergey Senozhatsky
2019-06-26 7:16 ` John Ogness
2019-06-26 7:45 ` Sergey Senozhatsky
2019-06-26 7:47 ` Petr Mladek
2019-06-26 7:59 ` Sergey Senozhatsky
2019-06-25 9:09 ` Sergey Senozhatsky
2019-06-18 11:12 ` Peter Zijlstra
2019-06-18 22:18 ` John Ogness
2019-06-18 11:22 ` Peter Zijlstra
2019-06-18 22:30 ` John Ogness
2019-06-19 10:46 ` Andrea Parri [this message]
2019-06-20 22:50 ` John Ogness
2019-06-21 12:16 ` Andrea Parri
2019-06-19 11:08 ` Peter Zijlstra
2019-06-18 11:47 ` Peter Zijlstra
2019-06-20 22:23 ` John Ogness
2019-06-26 22:40 ` Peter Zijlstra
2019-06-26 22:53 ` Peter Zijlstra
2019-06-28 9:50 ` John Ogness
2019-06-28 15:44 ` Peter Zijlstra
2019-06-28 16:07 ` Peter Zijlstra
2019-07-01 10:39 ` John Ogness
2019-07-01 14:10 ` Peter Zijlstra
2019-07-01 14:11 ` Peter Zijlstra
2019-06-29 21:05 ` Andrea Parri
2019-06-30 2:03 ` John Ogness
2019-06-30 14:08 ` Andrea Parri
2019-07-02 14:13 ` John Ogness
2019-06-26 22:47 ` Peter Zijlstra
2019-06-21 14:05 ` Petr Mladek
2019-06-24 8:33 ` John Ogness
2019-06-24 14:09 ` Petr Mladek
2019-06-25 13:29 ` John Ogness
2019-06-26 8:29 ` Petr Mladek
2019-06-26 9:09 ` John Ogness
2019-06-26 21:16 ` Peter Zijlstra
2019-06-26 21:43 ` John Ogness
2019-06-27 8:28 ` Petr Mladek
2019-07-04 10:33 ` [PATCH POC] printk_ringbuffer: Alternative implementation of lockless printk ringbuffer Petr Mladek
2019-07-04 14:59 ` John Ogness
2019-07-08 15:23 ` Petr Mladek
2019-07-09 1:34 ` John Ogness
2019-07-09 9:06 ` Petr Mladek
2019-07-09 10:21 ` John Ogness
2019-07-09 11:58 ` Petr Mladek
2019-08-14 3:46 ` John Ogness
2019-06-24 13:55 ` [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation John Ogness
2019-06-25 8:55 ` Sergey Senozhatsky
2019-06-25 9:19 ` John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 2/2] printk-rb: add test module John Ogness
2019-06-17 21:09 ` [RFC PATCH v2 0/2] printk: new ringbuffer implementation Thomas Gleixner
2019-06-18 7:15 ` Petr Mladek
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