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