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

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Date:   Tue, 28 Feb 2017 19:29:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
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
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

On Tue, Feb 28, 2017 at 10:24:44PM +0900, Byungchul Park wrote:
> On Tue, Feb 28, 2017 at 02:10:12PM +0100, Peter Zijlstra wrote:

> > > +/* For easy access to xhlock */
> > > +#define xhlock(t, i)		((t)->xhlocks + (i))
> > > +#define xhlock_prev(t, l)	xhlock(t, idx_prev((l) - (t)->xhlocks))
> > > +#define xhlock_curr(t)		xhlock(t, idx(t))
> > 
> > So these result in an xhlock pointer
> > 
> > > +#define xhlock_incr(t)		({idx(t) = idx_next(idx(t));})
> > 
> > This does not; which is confusing seeing how they share the same
> > namespace; also incr is weird.
> 
> OK.. Could you suggest a better name? xhlock_adv()? advance_xhlock()?
> And.. replace it with a function?

How about doing: xhlocks_idx++ ? That is, keep all the indexes as
regular u32 and only reduce the space when using them as index.

Also, I would write the loop:

> +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;
> +}

like:

#define xhlock(i)	current->xhlocks[i % MAX_XHLOCKS_NR]

	for (i = 0; i < MAX_XHLOCKS_NR; i++) {
		xhlock = xhlock(curr->xhlock_idx - i);

		/* ... */
	}

That avoids that horrible xhlock_prev() thing.