Re: Linux-kernel examples for LKMM recipes

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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