Re: XDP socket rings, and LKMM litmus tests

maranget
2021-03-03T20:56:54Z

> On 3 Mar 2021, at 18:12, Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Tue, Mar 02, 2021 at 03:50:19PM -0800, Paul E. McKenney wrote:
>> On Tue, Mar 02, 2021 at 04:14:46PM -0500, Alan Stern wrote:
>
>>> This result is wrong, apparently because of a bug in herd7. There
>>> should be control dependencies from each of the two loads in P0 to each
>>> of the two stores, but herd7 doesn't detect them.
>>>
>>> Maybe Luc can find some time to check whether this really is a bug and
>>> if it is, fix it.
>>
>> I agree that herd7's control dependency tracking could be improved.
>>
>> But sadly, it is currently doing exactly what I asked Luc to make it do,
>> which is to confine the control dependency to its "if" statement. But as
>> usual I wasn't thinking globally enough. And I am not exactly sure what
>> to ask for. Here a store to a local was control-dependency ordered after
>> a read, and so that should propagate to a read from that local variable.
>> Maybe treat local variables as if they were registers, so that from
>> herd7's viewpoint the READ_ONCE()s are able to head control-dependency
>> chains in multiple "if" statements?
>>
>> Thoughts?
>
> Local variables absolutely should be treated just like CPU registers, if
> possible. In fact, the compiler has the option of keeping local
> variables stored in registers.
>

And indeed local variables are treated as registers by herd7.

> (Of course, things may get complicated if anyone writes a litmus test
> that uses a pointer to a local variable, Especially if the pointer
> could hold the address of a local variable in one execution and a
> shared variable in another! Or if the pointer is itself a shared
> variable and is dereferenced in another thread!)
>
> But even if local variables are treated as non-shared storage locations,
> we should still handle this correctly. Part of the problem seems to lie
> in the definition of the to-r dependency relation; the relevant portion
> is:

In fact, I’d rather change the computation of “dep” here control-dependency “ctrl”. Notice that “ctrl” is computed by herd7 and present in the initial environment of the Cat interpreter.

I have made a PR to herd7 that performs the change. The commit message states the new definition.

>
> (dep ; [Marked] ; rfi)
>
> Here dep is the control dependency from the READ_ONCE to the
> local-variable store, and the rfi refers to the following load of the
> local variable. The problem is that the store to the local variable
> doesn't go in the Marked class, because it is notated as a plain C
> assignment. (And likewise for the following load.)
>
This is a related issue, I am not sure, but perhaps it can be formulated as
"should rfi and rf on registers behave the same?”

> Should we change the model to make loads from and stores to local
> variables always count as Marked?
>
> What should have happened if the local variable were instead a shared
> variable which the other thread didn't access at all? It seems like a
> weak point of the memory model that it treats these two things
> differently.
>
> Alan