linux-kernel - Re: Linux-kernel examples for LKMM recipes

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Date:   Tue, 17 Oct 2017 13:18:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     stern@...land.harvard.edu, will.deacon@....com,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        linux-kernel@...r.kernel.org
Subject: Re: Linux-kernel examples for LKMM recipes

On Thu, Oct 12, 2017 at 03:27:44PM +0200, Andrea Parri wrote:
> Hi Paul,
> 
> On Wed, Oct 11, 2017 at 03:32:30PM -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > At Linux Plumbers Conference, we got requests for a recipes document,
> > and a further request to point to actual code in the Linux kernel.
> > I have pulled together some examples for various litmus-test families,
> > as shown below.  The decoder ring for the abbreviations (ISA2, LB, SB,
> > MP, ...) is here:
> > 
> > 	https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf
> > 
> > This document is also checked into the memory-models git archive:
> > 
> > 	https://github.com/aparri/memory-model.git
> > 
> > I would be especially interested in simpler examples in general, and
> > of course any example at all for the cases where I was unable to find
> > any.  Thoughts?
> 
> Below are some examples we did discuss (at some point):
> 
> The comment in kernel/events/ring_buffer.c:perf_output_put_handle()
> describes instances of MP+wmb+rmb and LB+ctrl+mb.

I added this as an alternative for MP and as the example for LB.

> The comments in kernel/sched/core.c:try_to_wake_up() describes more
> instances of MP ("plus locking") and LB (see finish_lock_switch()).

This one looks a bit more messy, so I will set it aside, for the moment,
anyway.

> The comment in kernel/sched/core.c:task_rq_lock() describes an ins-
> tance of MP+wmb+addr-acqpo.

This one also looks a bit messy, so I am setting it aside as well.

> The comment in include/linux/wait.h:waitqueue_active() describes an
> instance of SB+mb+mb.

Very good, I took this as the generic pattern for the current pair
of SB examples.

> 63cae12bce986 ("perf/core: Fix sys_perf_event_open() vs. hotplug")
> describes an instance of W+RWC+porel+mb+mb.

Well, this one certainly is of historical interest.  After all, it might
well be the first Linux-kernel commit containing a litmus test.  ;-)

I put it in recipes-LKcode-63cae12bce986.txt and reference it from
recipes-LKcode.txt.

> [...]
> 
> I wish we could say "any barrier (explicit or implicit) in sources
> is accompanied by a comment mentioning the interested pattern...",
> but life is not always this simple. ;-)

Well, at least scripts/checkpatch.pl now complains if you try to add
a new comment-free barrier.  Not that these complaints are always
paid attention to...

							Thanx, Paul