Re: XDP socket rings, and LKMM litmus tests

[maranget]

> On 3 Mar 2021, at 18:12, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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