[PATCH v3] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[PATCH v3] 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@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>
---

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 | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index ee819a402b69..0b7e1925a673 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -464,9 +464,11 @@ 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
+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

Re: [PATCH v3] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[Alan Stern]

On Fri, Sep 02, 2022 at 09:13:40PM +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>

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

> ---
> 
> 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 | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> index ee819a402b69..0b7e1925a673 100644
> --- a/tools/memory-model/Documentation/explanation.txt
> +++ b/tools/memory-model/Documentation/explanation.txt
> @@ -464,9 +464,11 @@ 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
> +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;
> 

Re: [PATCH v3] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[Paul Heidekrüger]

On 3. Sep 2022, at 03:27, Alan Stern <stern@rowland.harvard.edu> wrote:

> On Fri, Sep 02, 2022 at 09:13:40PM +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>
> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> 
>> ---
>> 
>> 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 | 8 +++++---
>> 1 file changed, 5 insertions(+), 3 deletions(-)
>> 
>> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
>> index ee819a402b69..0b7e1925a673 100644
>> --- a/tools/memory-model/Documentation/explanation.txt
>> +++ b/tools/memory-model/Documentation/explanation.txt
>> @@ -464,9 +464,11 @@ 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
>> +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;

Hang on, shouldn't this read "a write event" instead of "another memory
access event"? Control dependencies only provide ordering from READ_ONCE to
WRITE_ONCE, not from READ_ONCE to (READ | WRITE)_ONCE?

Or am I missing something?

Many thanks,
Paul

Re: [PATCH v3] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[Alan Stern]

On Sat, Sep 03, 2022 at 01:41:34PM +0200, Paul Heidekrüger wrote:
> On 3. Sep 2022, at 03:27, Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> > On Fri, Sep 02, 2022 at 09:13:40PM +0000, Paul Heidekrüger wrote:
> >> +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
> >> +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;
> 
> Hang on, shouldn't this read "a write event" instead of "another memory
> access event"? Control dependencies only provide ordering from READ_ONCE to
> WRITE_ONCE, not from READ_ONCE to (READ | WRITE)_ONCE?
> 
> Or am I missing something?

Whoops, you're right.  Somehow I missed that.  Go ahead and change it; 
you can keep by S-O-B.

Alan