Re: [PATCH v3 07/15] lockdep: Implement crossrelease feature

From: Peter Zijlstra
Date: Wed Sep 14 2016 - 04:11:32 EST

On Wed, Sep 14, 2016 at 11:27:22AM +0900, Byungchul Park wrote:
> > Well, there is, its just not trivially observable. We must be able to
> > acquire a in order to complete b, therefore there is a dependency.
> No. We cannot say there is a dependency unconditionally. There can
> be a dependency or not.
> L a L a
> U a
> ~~~~~~~~~ what if serialized by something?

Well, there's no serialization in the example, so no what if.

> W b C b
> If something we don't recognize serializes locks, which ensures
> 'W b' happens after 'L a , U a' in the other context, then there's
> no dependency here.

Its not there.

> We should say 'b depends on a' in only case that the sequence
> 'W b and then L a and then C b, where last two ops are in same
> context' _actually_ happened at least once. Otherwise, it might
> add a false dependency.
> It's same as how original lockdep works with typical locks. It adds
> a dependency only when a lock is actually hit.

But since these threads are independently scheduled there is no point in
transferring the point in time thread A does W to thread B. There is no
relation there.

B could have already executed the complete or it could not yet have
started execution at all or anything in between, entirely random.

> > What does that mean? Any why? This is a random point in time without
> > actual meaning.
> It's not random point. We have to consider meaningful sequences among
> those which are globally observable. That's why we need to serialize
> those locks.

Serialize how? there is no serialization.

> For example,
> W b
> L a
> U a
> C b
> Once this sequence is observable globally, we can say 'It's possible to
> run in this sequence. Is this sequence problematic or not?'.
> L a
> U a
> W b
> C b
> If only this sequence can be observable, we should not assume
> this sequence can be changed. However once the former sequence
> happens, it has a possibility to hit the same sequence again later.
> So we can check deadlock possibility with the sequence,
> _not randomly_.

I still don't get it.

> We need to connect between the crosslock and the first lock among
> locks having been acquired since the crosslock was held.

Which can be _any_ lock in the history of that thread. It could be
rq->lock from getting the thread scheduled.

> Others will be
> connected each other by original lockdep.
> By the way, does my document miss this description? If so, sorry.
> I will check and update it.

I couldn't find anything useful, but then I could not understand most of
what was written, and I tried hard :-(