Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

[Peter Zijlstra]

On Wed, Mar 01, 2017 at 02:43:23PM +0900, Byungchul Park wrote:
> On Tue, Feb 28, 2017 at 02:40:18PM +0100, Peter Zijlstra wrote:
> > > +static int commit_xhlocks(struct cross_lock *xlock)
> > > +{
> > > +	struct task_struct *curr = current;
> > > +	struct hist_lock *xhlock_c = xhlock_curr(curr);
> > > +	struct hist_lock *xhlock = xhlock_c;
> > > +
> > > +	do {
> > > +		xhlock = xhlock_prev(curr, xhlock);
> > > +
> > > +		if (!xhlock_used(xhlock))
> > > +			break;
> > > +
> > > +		if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
> > > +			break;
> > > +
> > > +		if (same_context_xhlock(xhlock) &&
> > > +		    before(xhlock->prev_gen_id, xlock->hlock.gen_id) &&
> > > +		    !commit_xhlock(xlock, xhlock))
> > > +			return 0;
> > > +	} while (xhlock_c != xhlock);
> > > +
> > > +	return 1;
> > > +}
> > 
> > So I'm still struggling with prev_gen_id; is it an optimization or is it
> > required for correctness?
> 
> It's an optimization, but very essential and important optimization.
> 
>           in hlocks[]
>           ------------
>           A gen_id (4) --+
>                          | previous gen_id
>           B gen_id (3) <-+
>           C gen_id (3)
>           D gen_id (2)
> oldest -> E gen_id (1)
> 
>           in xhlocks[]
>           ------------
>        ^  A gen_id (4) prev_gen_id (3: B's gen id)
>        |  B gen_id (3) prev_gen_id (3: C's gen id)
>        |  C gen_id (3) prev_gen_id (2: D's gen id)
>        |  D gen_id (2) prev_gen_id (1: E's gen id)
>        |  E gen_id (1) prev_gen_id (NA)
> 
> Let's consider the case that the gen id of xlock to commit is 3.
> 
> In this case, it's engough to generate 'the xlock -> C'. 'the xlock -> B'
> and 'the xlock -> A' are unnecessary since it's covered by 'C -> B' and
> 'B -> A' which are already generated by original lockdep.
> 
> I use the prev_gen_id to avoid adding this kind of redundant
> dependencies. In other words, xhlock->prev_gen_id >= xlock->hlock.gen_id
> means that the previous lock in hlocks[] is able to handle the
> dependency on its commit stage.
> 

Aah, I completely missed it was against held_locks.

Hurm.. it feels like this is solving a problem we shouldn't be solving
though.

That is, ideally we'd already be able to (quickly) tell if a relation
exists or not, but given how the whole chain_hash stuff is build now, it
looks like we cannot.


Let me think about this a bit more.