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

Peter Zijlstra
2017-01-17T07:12:55Z

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:

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 */
}
}