On Fri, Oct 13, 2017 at 03:44:07PM -0400, Alan Stern wrote:
> 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.
Perhaps the recipes document should just baldly state that any execution
having only one thread and/or having only one variable will be fully
ordered?
Thanx, Paul
