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

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Date:   Tue, 17 Oct 2017 15:38:23 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:     Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will.deacon@....com>, <peterz@...radead.org>,
        <boqun.feng@...il.com>, <npiggin@...il.com>, <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Linux-kernel examples for LKMM recipes

On Tue, 17 Oct 2017, Paul E. McKenney wrote:

> How about this?
> 
> 0.	Simple special cases
> 
> 	If there is only one CPU on the one hand or only one variable
> 	on the other, the code will execute in order.  There are (as
> 	usual) some things to be careful of:
> 
> 	a.	There are some aspects of the C language that are
> 		unordered.  For example, the compiler can output code
> 		computing arguments of a multi-parameter function in
> 		any order it likes, or even interleaved if it so chooses.

That parses a little oddly.  I wouldn't agree that the compiler outputs
the code in any order it likes!

In fact, I wouldn't even mention the compiler at all.  Just say that
(with a few exceptions) the language doesn't specify the order in which
the arguments of a function or operation should be evaluated.  For
example, in the expression "f(x) + g(y)", the order in which f and g
are called is not defined; the object code is allowed to use either
order or even to interleave the computations.

> 	b.	Compilers are permitted to use the "as-if" rule.
> 		That is, a compiler can emit whatever code it likes,
> 		as long as the results appear just as if the compiler
> 		had followed all the relevant rules.  To see this,
> 		compiler with a high level of optimization and run
> 		the debugger on the resulting binary.

You might omit the last sentence.  Furthermore, if the accesses don't
use READ_ONCE/WRITE_ONCE then the code might not get the same result as
if it had executed in order (even for a single variable!), and if you
do use READ_ONCE/WRITE_ONCE then the compiler can't emit whatever code
it likes.

> 	c.	If there is only one variable but multiple CPUs, all
> 		accesses to that variable must be aligned and full sized.

I would say that the variable is what needs to be aligned, not the
accesses.  (Although, if the variable is aligned and all the accesses
are full sized, then they must necessarily be aligned as well.)

> 		Variables that straddle cachelines or pages void your
> 		full-ordering warranty, as do undersized accesses that
> 		load from or store to only part of the variable.

How can a variable straddle pages without also straddling cache lines?

Alan