Re: Linux-kernel examples for LKMM recipes

[Alan Stern]

On Wed, 11 Oct 2017, Paul E. McKenney wrote:

> This document lists the litmus-test patterns that we have been discussing,
> along with examples from the Linux kernel.  This is intended to feed into
> the recipes document.  All examples are from v4.13.
> 
> 0.	Single-variable SC.
> 
> 	a.	Within a single CPU, the use of the ->dynticks_nmi_nesting
> 		counter by rcu_nmi_enter() and rcu_nmi_exit() qualifies
> 		(see kernel/rcu/tree.c).  The counter is accessed by
> 		interrupts and NMIs as well as by process-level code.
> 		This counter can be accessed by other CPUs, but only
> 		for debug output.

I'm not sure that single-variable SC can really be represented by an 
example.  It gets used literally all over the kernel -- it's such a 
large part of the way we think about computer programs that we rely on 
it unconsciously.

For example, the very first function in the very first C source file 
in the kernel/ directory (namely, check_free_space() in kernel/acct.c) 
includes this code:

        if (acct->active) {
                u64 suspend = sbuf.f_blocks * SUSPEND;
                do_div(suspend, 100);

How do we know that the value which gets divided by 100 is
sbuf.f_blocks * SUSPEND and not the random garbage which was stored in
suspend's memory location before it was initialized?  Answer:
per-variable SC.

Okay, maybe that's not really applicable, since it doesn't involve
accesses to shared memory.  Here's an example that does.  
get_futex_key() in kernel/futex.c calls READ_ONCE(page->mapping) twice.  
How do we know that the value retrieved by the second call was not
stored _earlier_ than the value retrieved by the first call?  
Per-variable SC.

> 	b.	Between CPUs, I would put forward the ->dflags
> 		updates, but this is anything but simple.  But maybe
> 		OK for an illustration?

Pretty much any code that accesses the same shared variable twice on
the same CPU could be an example of per-variable SC.  But I don't think 
people would learn much by studying such examples.

Alan