On Mon, Apr 24, 2017 at 12:04:12PM +0900, Byungchul Park wrote:
> On Wed, Apr 19, 2017 at 07:19:54PM +0200, Peter Zijlstra wrote:
> > > +/*
> > > + * For crosslock.
> > > + */
> > > +static int add_xlock(struct held_lock *hlock)
> > > +{
> > > + struct cross_lock *xlock;
> > > + unsigned int gen_id;
> > > +
> > > + if (!graph_lock())
> > > + return 0;
> > > +
> > > + xlock = &((struct lockdep_map_cross *)hlock->instance)->xlock;
> > > +
> > > + gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
> > > + xlock->hlock = *hlock;
> > > + xlock->hlock.gen_id = gen_id;
> > > + graph_unlock();
> >
> > What does graph_lock protect here?
>
> Modifying xlock(not xhlock) instance should be protected with graph_lock.
> Don't you think so?
Ah, right you are. I think I got confused between our xhlock (local)
array and the xlock instance thing. The latter needs protection to
serialize concurrent acquires.
> > > +static int commit_xhlocks(struct cross_lock *xlock)
> > > +{
> > > + unsigned int cur = current->xhlock_idx;
> > > + unsigned int i;
> > > +
> > > + if (!graph_lock())
> > > + return 0;
> > > +
> > > + for (i = cur - 1; !xhlock_same(i, cur); i--) {
> > > + struct hist_lock *xhlock = &xhlock(i);
> >
> > *blink*, you mean this?
> >
> > for (i = 0; i < MAX_XHLOCKS_NR; i++) {
> > struct hist_lock *xhlock = &xhlock(cur - i);
>
> I will change the loop to this form.
>
> > Except you seem to skip over the most recent element (@cur), why?
>
> Currently 'cur' points to the next *free* slot.
Well, there's no such thing has a 'free' slot, its a _ring_ buffer.
But:
+static void add_xhlock(struct held_lock *hlock)
+{
+ unsigned int idx = current->xhlock_idx++;
+ struct hist_lock *xhlock = &xhlock(idx);
Yes, I misread that. Then '0' has the oldest entry, which is slightly
weird. Should we change that?
> > > +
> > > + if (!xhlock_used(xhlock))
> > > + break;
> > > +
> > > + if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
> > > + break;
> > > +
> > > + if (same_context_xhlock(xhlock) &&
> > > + !commit_xhlock(xlock, xhlock))
> >
> > return with graph_lock held?
>
> No. When commit_xhlock() returns 0, the lock was already unlocked.
Please add a comment, because I completely missed that. That's at least
2 functions deeper.
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