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

Byungchul Park
2017-03-01T05:44:17Z

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.