Messages in this thread | | | Date | Fri, 13 Oct 2017 13:00:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Linux-kernel examples for LKMM recipes |
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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
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