Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature

Byungchul Park
2017-01-17T08:05:11Z

On Tue, Jan 17, 2017 at 08:12:20AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 17, 2017 at 11:05:42AM +0900, Byungchul Park wrote:
> > On Mon, Jan 16, 2017 at 04:10:01PM +0100, Peter Zijlstra wrote:
> > > On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote:
>
> > > > +
> > > > + /*
> > > > + * Whenever irq happens, these are updated so that we can
> > > > + * distinguish each irq context uniquely.
> > > > + */
> > > > + unsigned int hardirq_id;
> > > > + unsigned int softirq_id;
> > >
> > > An alternative approach would be to 'unwind' or discard all historical
> > > events from a nested context once we exit it.
> >
> > That's one of what I considered. However, it would make code complex to
> > detect if pend_lock ring buffer was wrapped.
>
> I'm not sure I see the need for detecting that...
>
> > >
> > > After all, all we care about is the history of the release context, once
> > > the context is gone, we don't care.
> >
> > We must care it and decide if the next plock in the ring buffer might be
> > valid one or not.
>
> So I was thinking this was an overwriting ring buffer; something like
> so:

OK. I am making code just overwrite ring buffer when overflowing it,
instead of warning the situation.

Thank you very much.

>
> struct pend_lock plocks[64];
> unsigned int plocks_idx;
>
> static void plocks_add(..)
> {
> unsigned int idx = (plocks_idx++) % 64;
>
> plocks[idx] = ...;
> }
>
> static void plocks_close_context(int ctx)
> {
> for (i = 0; i < 64; i++) {
> int idx = (plocks_idx - 1) % 64;
> if (plocks[idx].ctx != ctx)
> break;
>
> plocks_idx--;
> }
> }
>
> Similarly for the release, it need only look at 64 entries and terminate
> early if the generation number is too old.
>
> static void plocks_release(unsigned int gen)
> {
> for (i = 0; i < 64; i++) {
> int idx = (plocks_idx - 1 - i) % 64;
> if ((int)(plocks[idx].gen_id - gen) < 0)
> break;
>
> /* do release muck */
> }
> }