Re: [PATCH] tools/memory-model: Explain syntactic and semantic dependencies

[Paul E. McKenney]

On Tue, Feb 01, 2022 at 01:53:14PM -0500, Alan Stern wrote:
> On Tue, Feb 01, 2022 at 10:02:39AM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 27, 2022 at 04:11:48PM -0500, Alan Stern wrote:
> > > Paul Heidekrüger pointed out that the Linux Kernel Memory Model
> > > documentation doesn't mention the distinction between syntactic and
> > > semantic dependencies.  This is an important difference, because the
> > > compiler can easily break dependencies that are only syntactic, not
> > > semantic.
> > > 
> > > This patch adds a few paragraphs to the LKMM documentation explaining
> > > these issues and illustrating how they can matter.
> > > 
> > > Suggested-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx>
> > > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > 
> > > ---
> > > 
> > > 
> > > [as1970]
> > > 
> > > 
> > >  tools/memory-model/Documentation/explanation.txt |   47 +++++++++++++++++++++++
> > >  1 file changed, 47 insertions(+)
> > > 
> > > Index: usb-devel/tools/memory-model/Documentation/explanation.txt
> > > ===================================================================
> > > --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> > > +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> > > @@ -485,6 +485,53 @@ have R ->po X.  It wouldn't make sense f
> > >  somehow on a value that doesn't get loaded from shared memory until
> > >  later in the code!
> > >  
> > > +Here's a trick question: When is a dependency not a dependency?  Answer:
> > > +When it is purely syntactic rather than semantic.  We say a dependency
> > > +between two accesses is purely syntactic if the second access doesn't
> > > +actually depend on the result of the first.  Here is a trivial example:
> > > +
> > > +	r1 = READ_ONCE(x);
> > > +	WRITE_ONCE(y, r1 * 0);
> > > +
> > > +There appears to be a data dependency from the load of x to the store of
> > > +y, since the value to be stored is computed from the value that was
> > > +loaded.  But in fact, the value stored does not really depend on
> > > +anything since it will always be 0.  Thus the data dependency is only
> > > +syntactic (it appears to exist in the code) but not semantic (the second
> > > +access will always be the same, regardless of the value of the first
> > > +access).  Given code like this, a compiler could simply eliminate the
> > > +load from x, which would certainly destroy any dependency.
> > 
> > Are you OK with that last sentence reading as follows?
> > 
> > 	Given code like this, a compiler could simply discard the value
> > 	return by the load from x, which would certainly destroy any
> 
> s/return/returned/

Good eyes!

> > 	dependency.
> > 
> > My concern with the original is that it might mislead people into thinking
> > that compilers can omit volatile loads.
> 
> Yes, good point.  Should we also tack on something like this?
> 
> 	(The compiler is not permitted to eliminate entirely the load 
> 	generated for a READ_ONCE() -- that's one of the nice properties 
> 	of READ_ONCE() -- but it is allowed to ignore the load's value.)

Please!

> > > +
> > > +(It's natural to object that no one in their right mind would write code
> > > +like the above.  However, macro expansions can easily give rise to this
> > > +sort of thing, in ways that generally are not apparent to the
> > > +programmer.)
> > > +
> > > +Another mechanism that can give rise to purely syntactic dependencies is
> > > +related to the notion of "undefined behavior".  Certain program behaviors
> > > +are called "undefined" in the C language specification, which means that
> > > +when they occur there are no guarantees at all about the outcome.
> > > +Consider the following example:
> > > +
> > > +	int a[1];
> > > +	int i;
> > > +
> > > +	r1 = READ_ONCE(i);
> > > +	r2 = READ_ONCE(a[r1]);
> > > +
> > > +Access beyond the end or before the beginning of an array is one kind of
> > > +undefined behavior.  Therefore the compiler doesn't have to worry about
> > > +what will happen if r1 is nonzero, and it can assume that r1 will always
> > > +be zero without actually loading anything from i.
> > 
> > And similarly here:
> > 
> > 	... and it can assume that r1 will always be zero regardless of
> > 	the value actually loaded from i.
> 
> Right.
> 
> > > +                                                   (If the assumption
> > > +turns out to be wrong, the resulting behavior will be undefined anyway
> > > +so the compiler doesn't care!)  Thus the load from i can be eliminated,
> > > +breaking the address dependency.
> 
> This also should be changed:
> 
> 	Thus the value from the load can be discarded, breaking the 
> 	address dependency.

Again, good eyes!

> > > +
> > > +The LKMM is unaware that purely syntactic dependencies are different
> > > +from semantic dependencies and therefore mistakenly predicts that the
> > > +accesses in the two examples above will be ordered.  This is another
> > > +example of how the compiler can undermine the memory model.  Be warned.
> > > +
> > >  
> > >  THE READS-FROM RELATION: rf, rfi, and rfe
> > >  -----------------------------------------
> > 
> > Looks great otherwise, and thank you all for your work on this!
> > 
> > Alan, would you like me to pull this in making those two changes and
> > applying Akira's Reviewed-by, or would you prefer to send another version?
> 
> I'll send a new version.

Very good, looking forward to it!

							Thanx, Paul

> > For that matter, am I off base in my suggested changes.
> 
> Not at all.  Thanks.
> 
> Alan