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>
Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
---

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..0bca50cac5f4 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 another memory access event Y are linked by
+a control dependency if Y syntactically lies within an arm of an if,
+else or switch statement and the condition guarding Y is either data or
+address-dependent on X.  Simple example:

 	int x, y;

--
2.35.1

On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekrüger wrote:
> 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>
> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>

Hearing no objections, I reverted the old version and replaced it
with this version.  Thank you both!

							Thanx, Paul

> ---
> 
> 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..0bca50cac5f4 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 another memory access event Y are linked by
> +a control dependency if Y syntactically lies within an arm of an if,
> +else or switch statement and the condition guarding Y is either data or
> +address-dependent on X.  Simple example:
> 
>  	int x, y;
> 
> --
> 2.35.1
> 

On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@kernel.org> wrote:

> On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekrüger wrote:
>> 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>
>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
>> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
> 
> Hearing no objections, I reverted the old version and replaced it
> with this version.  Thank you both!
> 
> 							Thanx, Paul

Oh, wait, there was further discussion [1, 2], and we finally agreed on [3].
So [3] is the final version.

I think me sending a v2 immediately after the v1 led to this out-of-order
discussion - sorry!

Many thanks,
Paul

[1]: https://lore.kernel.org/all/663d568d-a343-d44b-d33d-29998bff8f70@joelfernandes.org/
[2]: https://lore.kernel.org/all/D7E3D42D-2ABE-4D16-9DCA-0605F0C84F7D@in.tum.de/
[3]: https://lore.kernel.org/all/20220903165718.4186763-1-paul.heidekrueger@in.tum.de/


>> ---
>> 
>> 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..0bca50cac5f4 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 another memory access event Y are linked by
>> +a control dependency if Y syntactically lies within an arm of an if,
>> +else or switch statement and the condition guarding Y is either data or
>> +address-dependent on X.  Simple example:
>> 
>> 	int x, y;
>> 
>> --
>> 2.35.1

On Mon, Sep 12, 2022 at 02:38:58PM +0100, Paul Heidekrüger wrote:
> On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> > On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekrüger wrote:
> >> 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>
> >> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
> >> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
> > 
> > Hearing no objections, I reverted the old version and replaced it
> > with this version.  Thank you both!
> > 
> > 							Thanx, Paul
> 
> Oh, wait, there was further discussion [1, 2], and we finally agreed on [3].
> So [3] is the final version.
> 
> I think me sending a v2 immediately after the v1 led to this out-of-order
> discussion - sorry!

My bad, and thank you for checking and letting me know!

I have reverted to the proper state.

							Thanx, Paul

> Many thanks,
> Paul
> 
> [1]: https://lore.kernel.org/all/663d568d-a343-d44b-d33d-29998bff8f70@joelfernandes.org/
> [2]: https://lore.kernel.org/all/D7E3D42D-2ABE-4D16-9DCA-0605F0C84F7D@in.tum.de/
> [3]: https://lore.kernel.org/all/20220903165718.4186763-1-paul.heidekrueger@in.tum.de/
> 
> 
> >> ---
> >> 
> >> 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..0bca50cac5f4 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 another memory access event Y are linked by
> >> +a control dependency if Y syntactically lies within an arm of an if,
> >> +else or switch statement and the condition guarding Y is either data or
> >> +address-dependent on X.  Simple example:
> >> 
> >> 	int x, y;
> >> 
> >> --
> >> 2.35.1
> 
>