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
