Commit fc13b476 authored Sep 03, 2022 by Paul Heidekrüger Committed by Paul E. McKenney Oct 18, 2022

tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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@in.tum.de/


Cc: Marco Elver <elver@google.com>
Cc: Charalampos Mainas <charalampos.mainas@gmail.com>
Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de>
Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl>
Cc: Martin Fink <martin.fink@in.tum.de>
Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

parent 9abf2313

tools/memory-model/Documentation/explanation.txt

+4 −3

Original line number	Diff line number	Diff line
		@@ -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;