Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Jonas Oberhauser
2023-01-17T19:26:17Z

On 1/17/2023 6:43 PM, Paul E. McKenney wrote:
> On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
>> On Tue, Jan 17, 2023 at 07:14:16AM -0800, Paul E. McKenney wrote:
>>> On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote:
>>>> This was reminiscent of old discussions, in fact, we do have:
>>>>
>>>> [tools/memory-model/Documentation/litmus-tests.txt]
>>>>
>>>> e. Although sleepable RCU (SRCU) is now modeled, there
>>>> are some subtle differences between its semantics and
>>>> those in the Linux kernel. For example, the kernel
>>>> might interpret the following sequence as two partially
>>>> overlapping SRCU read-side critical sections:
>>>>
>>>> 1 r1 = srcu_read_lock(&my_srcu);
>>>> 2 do_something_1();
>>>> 3 r2 = srcu_read_lock(&my_srcu);
>>>> 4 do_something_2();
>>>> 5 srcu_read_unlock(&my_srcu, r1);
>>>> 6 do_something_3();
>>>> 7 srcu_read_unlock(&my_srcu, r2);
>>>>
>>>> In contrast, LKMM will interpret this as a nested pair of
>>>> SRCU read-side critical sections, with the outer critical
>>>> section spanning lines 1-7 and the inner critical section
>>>> spanning lines 3-5.
>>>>
>>>> This difference would be more of a concern had anyone
>>>> identified a reasonable use case for partially overlapping
>>>> SRCU read-side critical sections. For more information
>>>> on the trickiness of such overlapping, please see:
>>>> https://paulmck.livejournal.com/40593.html
>>> Good point, if we do change the definition, we also need to update
>>> this documentation.
>>>
>>>> More recently/related,
>>>>
>>>> https://lore.kernel.org/lkml/20220421230848.GA194034@paulmck-ThinkPad-P17-Gen-1/T/#m2a8701c7c377ccb27190a6679e58b0929b0b0ad9
>>> It would not be a bad thing for LKMM to be able to show people the
>>> error of their ways when they try non-nested partially overlapping SRCU
>>> read-side critical sections. Or, should they find some valid use case,
>>> to help them prove their point. ;-)
>> Isn't it true that the current code will flag srcu-bad-nesting if a
>> litmus test has non-nested overlapping SRCU read-side critical sections?
> Now that you mention it, it does indeed, flagging srcu-bad-nesting.
>
> Just to see if I understand, different-values yields true if the set
> contains multiple elements with the same value mapping to different
> values. Or, to put it another way, if the relation does not correspond
> to a function.
>
> Or am I still missing something?

based on https://lkml.org/lkml/2019/1/10/155:
I think different-values(r) is the same as r \ same-values, where
same-values links all reads and writes that have the same value (e.g.,
"write 5 to x" and "read 5 from y").

With this in mind, I think the idea is to 1) forbid partial overlap, and
using the different-values to 2) force them to provide the appropriate
value.
This works because apparently srcu-lock is a read and srcu-unlock is a
write, so in case of
int r1 = srcu-lock(&ss);   ==>  Read(&ss, x), r1 := x
...
srcu-unlock(&ss, r1);  ==> Write(&ss, r1), which is Write(&ss, x)

This guarantees that the read and write have the same value, hence
different-values(...) will be the empty relation, and so no flag.

>
>> And if it is true, is there any need to change the memory model at this
>> point?
>>
>> (And if it's not true, that's most likely due to a bug in herd7.)
> Agreed, changes must wait for SRCU support in herd7.
>
> At which point something roughly similar to this might work?
>
> let srcu-rscs = return_value(Srcu-lock) ; (dep | rfi)* ;
> parameter(Srcu-unlock, 2)

I would like instead to be able to give names to the arguments of events
that become dependency relations, like
event srcu_unlock(struct srcu_struct *srcu_addr, struct srcu_token
*srcu_data)
and then
let srcu-rscs = [Srcu-lock] ; srcu_data ; (data; rfi)*

Personally I would also like to not have Linux-specific primitives in
herd7/cat, that means that to understand LKMM you also need to
understand the herd7 tool, and sounds quite brittle.

I would prefer if herd7 had some means to define custom
events/instructions and uninterpreted relations between them, like

relation rf : [write] x [read]
[read] <= range(rf)
empty rf ;rf^-1 \ id

and some way to say
[read] ; .return <= rf^-1 ; .data
(where .return is a functional relation relating every event to the
value it returns, and .xyz is the functional relation relating every
event to the value of its argument xyz).