Re: [PATCH v2] tools/memory-model: Explain syntactic and semantic dependencies

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Feb 01, 2022 at 02:00:08PM -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@in.tum.de>
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

Very good, queued and pushed with Akira's Reviewed-by.

Thank you all!

							Thanx, Paul

> ---
> 
> v2:	Incorporate changes suggested by Paul McKenney, along with a few
> 	other minor edits.
>  
> 
> [as1970b]
> 
> 
>  tools/memory-model/Documentation/explanation.txt |   51 +++++++++++++++++++++++
>  1 file changed, 51 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 +485,57 @@ 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 discard
> +the value returned by the load from x, which would certainly destroy
> +any dependency.  (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.)
> +
> +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 often are not apparent to the
> +programmer.
> +
> +Another mechanism that can lead 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 regardless of the value actually loaded from i.
> +(If the assumption turns out to be wrong the resulting behavior will
> +be undefined anyway, so the compiler doesn't care!)  Thus the value
> +from the load can be discarded, breaking the address dependency.
> +
> +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
>  -----------------------------------------