Re: (Non-) Ctrl Dependency in litmus-tests.txt?

From: Alan Stern
Date: Fri Jun 03 2022 - 10:48:22 EST


On Fri, Jun 03, 2022 at 04:12:37PM +0200, Paul Heidekrüger wrote:
> Hi all,
>
> I was going through litmus-tests.txt and came across the following:
>
> > LIMITATIONS
> > ===========
> >
> > Limitations of the Linux-kernel memory model (LKMM) include:
> >
> > 1.Compiler optimizations are not accurately modeled. Of course,
> > the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
> > ability to optimize, but under some circumstances it is possible
> > for the compiler to undermine the memory model. For more
> > information, see Documentation/explanation.txt (in particular,
> > the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING"
> > sections).
> >
> > Note that this limitation in turn limits LKMM's ability to
> > accurately model address, control, and data dependencies.
> > For example, if the compiler can deduce the value of some variable
> > carrying a dependency, then the compiler can break that dependency
> > by substituting a constant of that value.
> >
> > Conversely, LKMM sometimes doesn't recognize that a particular
> > optimization is not allowed, and as a result, thinks that a
> > dependency is not present (because the optimization would break it).
> > The memory model misses some pretty obvious control dependencies
> > because of this limitation. A simple example is:
> >
> > r1 = READ_ONCE(x);
> > if (r1 == 0)
> > smp_mb();
> > WRITE_ONCE(y, 1);
> >
> > There is a control dependency from the READ_ONCE to the WRITE_ONCE,
> > even when r1 is nonzero, but LKMM doesn't realize this and thinks
> > that the write may execute before the read if r1 != 0. (Yes, that
> > doesn't make sense if you think about it, but the memory model's
> > intelligence is limited.)
>
> I'm unclear as to why the documentation sees a control dependency from
> the READ_ONCE() to the WRITE_ONCE() here.
>
> Quoting from explanation.txt:
> > Finally, a read event and another memory access event are linked by a
> > control dependency if the value obtained by the read affects whether
> > the second event is executed at all.
>
> Architectures might consider this control-dependent, yes, but since the
> value of the if condition does not affect whether the WRITE_ONCE() is
> executed at all, I'm not sure why this should be considered
> control-dependent in LKMM?
>
> I might have another question about explanation.txt's definition of
> control dependencies as per above, but will address it more thoroughly
> in another email :-)

You're right; strictly speaking this isn't a control dependency. In
fact it's not a dependency at all, just an ordering restriction that's
connected with a conditional test.

If you would like to submit a patch updating the text, please feel free
to do so.

Alan