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tools/memory-model: Explain syntactic and semantic dependencies

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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>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Alan Stern 2022-02-01 14:00:08 -05:00 committed by Paul E. McKenney
parent da123016ca
commit e2b665f612
1 changed files with 51 additions and 0 deletions
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51
tools/memory-model/Documentation/explanation.txt
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@ -485,6 +485,57 @@ have R ->po X. It wouldn't make sense for a computation to depend
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
-----------------------------------------