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

Jonas Oberhauser
2023-01-24T15:55:34Z

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).

Anyways, not much to be done about it right now.

best wishes, jonas