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

Byungchul Park
2017-03-01T12:29:24Z

On Wed, Mar 01, 2017 at 11:43:28AM +0100, Peter Zijlstra wrote:
> On Wed, Mar 01, 2017 at 04:21:28PM +0900, Byungchul Park wrote:
> > On Tue, Feb 28, 2017 at 07:15:47PM +0100, Peter Zijlstra wrote:
> > > On Wed, Jan 18, 2017 at 10:17:32PM +0900, Byungchul Park wrote:
> > > > + /*
> > > > + * Each work of workqueue might run in a different context,
> > > > + * thanks to concurrency support of workqueue. So we have to
> > > > + * distinguish each work to avoid false positive.
> > > > + *
> > > > + * TODO: We can also add dependencies between two acquisitions
> > > > + * of different work_id, if they don't cause a sleep so make
> > > > + * the worker stalled.
> > > > + */
> > > > + unsigned int work_id;
> > >
> > > > +/*
> > > > + * Crossrelease needs to distinguish each work of workqueues.
> > > > + * Caller is supposed to be a worker.
> > > > + */
> > > > +void crossrelease_work_start(void)
> > > > +{
> > > > + if (current->xhlocks)
> > > > + current->work_id++;
> > > > +}
> > >
> > > So what you're trying to do with that 'work_id' thing is basically wipe
> > > the entire history when we're at the bottom of a context.
> >
> > Sorry, but I do not understand what you are trying to say.
> >
> > What I was trying to do with the 'work_id' is to distinguish between
> > different works, which will be used to check if history locks were held
> > in the same context as a release one.
>
> The effect of changing work_id is that history disappears, yes? That is,
> by changing it, all our hist_locks don't match the context anymore and
> therefore we have no history.

Right. Now I understood your words.

> This is a useful operation.
>
> You would want to do this at points where you know there will not be any
> dependencies on prior action, and typically at the same points we want
> to not be holding any locks.
>
> Hence my term: 'bottom of a context', referring to an empty (held) lock
> stack.

Right.

> I would say this needs to be done for all 'work-queue' like things, and

Of course.

> there are quite a few outside of the obvious ones, smpboot threads and
> many other kthreads fall into this category.

Where can I check those?

> Similarly the return to userspace point that I already mentioned.
>
> I would propose something like:
>
> lockdep_assert_empty();
>
> Or something similar, which would verify the lock stack is indeed empty
> and wipe our entire hist_lock buffer when cross-release is enabled.

Right. I should do that.

> > > Which is a useful operation, but should arguably also be done on the
> > > return to userspace path. Any historical lock from before the current
> > > syscall is irrelevant.

Let me think more. It looks not a simple problem.

> >
> > Sorry. Could you explain it more?
>
> Does the above make things clear?

Perfect. Thank you very much.