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/[email protected]/
Cc: Marco Elver <[email protected]>
Cc: Charalampos Mainas <[email protected]>
Cc: Pramod Bhatotia <[email protected]>
Cc: Soham Chakraborty <[email protected]>
Cc: Martin Fink <[email protected]>
Signed-off-by: Paul Heidekrüger <[email protected]>
Co-developed-by: Alan Stern <[email protected]>
---
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 Heidekr?ger 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/[email protected]/
> Cc: Marco Elver <[email protected]>
> Cc: Charalampos Mainas <[email protected]>
> Cc: Pramod Bhatotia <[email protected]>
> Cc: Soham Chakraborty <[email protected]>
> Cc: Martin Fink <[email protected]>
> Signed-off-by: Paul Heidekr?ger <[email protected]>
> Co-developed-by: Alan Stern <[email protected]>
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 <[email protected]> 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/[email protected]/
>> Cc: Marco Elver <[email protected]>
>> Cc: Charalampos Mainas <[email protected]>
>> Cc: Pramod Bhatotia <[email protected]>
>> Cc: Soham Chakraborty <[email protected]>
>> Cc: Martin Fink <[email protected]>
>> Signed-off-by: Paul Heidekrüger <[email protected]>
>> Co-developed-by: Alan Stern <[email protected]>
>
> 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/[email protected]/
[2]: https://lore.kernel.org/all/[email protected]/
[3]: https://lore.kernel.org/all/[email protected]/
>> ---
>>
>> 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 Heidekr?ger wrote:
> On 12. Sep 2022, at 12:38, Paul E. McKenney <[email protected]> wrote:
>
> > On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekr?ger 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/[email protected]/
> >> Cc: Marco Elver <[email protected]>
> >> Cc: Charalampos Mainas <[email protected]>
> >> Cc: Pramod Bhatotia <[email protected]>
> >> Cc: Soham Chakraborty <[email protected]>
> >> Cc: Martin Fink <[email protected]>
> >> Signed-off-by: Paul Heidekr?ger <[email protected]>
> >> Co-developed-by: Alan Stern <[email protected]>
> >
> > 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/[email protected]/
> [2]: https://lore.kernel.org/all/[email protected]/
> [3]: https://lore.kernel.org/all/[email protected]/
>
>
> >> ---
> >>
> >> 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
>
>