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

Byungchul Park
2017-03-03T00:32:58Z

On Thu, Mar 02, 2017 at 02:40:31PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 01, 2017 at 01:28:43PM +0100, Peter Zijlstra wrote:
> > On Wed, Mar 01, 2017 at 02:43:23PM +0900, Byungchul Park wrote:
>
> > > It's an optimization, but very essential and important optimization.
>
> Since its not for correctness, please put it in a separate patch with a
> good Changelog, the below would make a good beginning of that.

OK. I will do it.

> Also, I feel, the source comments can be improved.
>
> > > 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.

> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index a95e5d1..7baea89 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1860,6 +1860,17 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
> }
> }
>
> + /*
> + * Is the <prev> -> <next> redundant?
> + */
> + this.class = hlock_class(prev);
> + this.parent = NULL;
> + ret = check_noncircular(&this, hlock_class(next), &target_entry);
> + if (!ret) /* exists, redundant */
> + return 2;
> + if (ret < 0)
> + return print_bfs_bug(ret);
> +
> if (!*stack_saved) {
> if (!save_trace(&trace))
> return 0;

This whoud be very nice if you allow to add this code. However, prev_gen_id
thingy is still useful, the code above can achieve it though. Agree?