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

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Date:   Wed, 1 Mar 2017 14:43:23 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com, kernel-team@....com
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

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.