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

Alan Stern
2023-01-24T17:22:22Z

On Tue, Jan 24, 2023 at 04:54:42PM +0100, Jonas Oberhauser wrote:
>
>
> On 1/19/2023 5:41 PM, Alan Stern wrote:
> > On Thu, Jan 19, 2023 at 12:22:50PM +0100, Jonas Oberhauser wrote:
> > >
> > > On 1/19/2023 3:28 AM, Alan Stern wrote:
> > > > > This is a permanent error; I've given up. Sorry it didn't
> > > > work out.
> > > [It seems the e-mail still reached me through the mailing list]
> > [For everyone else, Jonas is referring to the fact that the last two
> > emails I sent to his huaweicloud.com address could not be delivered, so
> > I copied them off-list to his huawei.com address.]
> >
> > > > > I consider that a hack though and don't like it.
> > > > It _is_ a bit of a hack, but not a huge one. srcu_read_lock() really
> > > > is a lot like a load, in that it returns a value obtained by reading
> > > > something from memory (along with some other operations, though, so it
> > > > isn't a simple straightforward read -- perhaps more like an
> > > > atomic_inc_return_relaxed).
> > > The issue I have with this is that it might create accidental ordering. How
> > > does it behave when you throw fences in the mix?
> > I think this isn't going to be a problem. Certainly any real
> > implementation of scru_read_lock() is going to involve some actual load
> > operations, so any unintentional ordering caused by fences will also
> > apply to real executions. Likewise for srcu_read_unlock and store
> > operations.
>
> Note that there may indeed be reads in the implementation, but most likely
> not from the srcu_read_unlock()s of other threads. Most probably from the
> synchronize_srcu() calls. So the rfe edges being added are probably not
> corresponding to any rfe edges in the implementation.
>
> That said, I believe there may indeed not be any restrictions in behavior
> caused by this, because any code that relies on the order being a certain
> thing would need to use some other ordering mechanism, and that would
> probably restrict the behavior anyways.
>
> It does have the negative side-effect of creating an explosion of
> permutations though, by ordering all unlocks() in a total way and also
> sometimes allowing multiple options for each lock() (e.g., lock();unlock()
> || lock();unlock()  has 4 executions instead of 1).

That's true. It would be nice if there was a class of write-like events
which couldn't be read from and didn't contribute to the coherence
ordering.

Alan

> Anyways, not much to be done about it right now.
>
> best wishes, jonas
>