[PATCH v4] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[Paul Heidekrüger]

The current informal control dependency definition in explanation.txt is
too broad and, as discussed, needs to be updated.

Consider the following example:

> if(READ_ONCE(x))
>   return 42;
>
> WRITE_ONCE(y, 42);
>
> return 21;

The read event determines whether the write event will be executed "at all"
- as per the current definition - but the formal LKMM does not recognize
this as a control dependency.

Introduce a new definition which includes the requirement for the second
memory access event to syntactically lie within the arm of a non-loop
conditional.

Link: https://lore.kernel.org/all/20220615114330.2573952-1-paul.heidekrueger@xxxxxxxxx/
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Charalampos Mainas <charalampos.mainas@xxxxxxxxx>
Cc: Pramod Bhatotia <pramod.bhatotia@xxxxxxxxx>
Cc: Soham Chakraborty <s.s.chakraborty@xxxxxxxxxx>
Cc: Martin Fink <martin.fink@xxxxxxxxx>
Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx>
---

v4:
- Replace "a memory access event" with "a write event"

v3:
- Address Alan and Joel's feedback re: the wording around switch statements
and the use of "guarding"

v2:
- Fix typos
- Fix indentation of code snippet

v1:
@Alan, since I got it wrong the last time, I'm adding you as a co-developer
after my SOB. I'm sorry if this creates extra work on your side due to you
having to resubmit the patch now with your SOB if I understand correctly,
but since it's based on your wording from the other thread, I definitely
wanted to give you credit.

 tools/memory-model/Documentation/explanation.txt | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index ee819a402b69..11a1d2d4f681 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -464,9 +464,10 @@ to address dependencies, since the address of a location accessed
 through a pointer will depend on the value read earlier from that
 pointer.

-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.  Simple example:
+Finally, a read event X and a write event Y are linked by a control
+dependency if Y syntactically lies within an arm of an if statement and
+X affects the evaluation of the if condition via a data or address
+dependency (or similarly for a switch statement).  Simple example:

 	int x, y;

--
2.35.1