On Thu, Aug 24, 2017 at 04:02:40PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 24, 2017 at 11:18:40AM +0900, Byungchul Park wrote:
> > On Wed, Aug 23, 2017 at 01:58:47PM +0200, Peter Zijlstra wrote:
>
> > > Also, unconditinoally switching to recursive-read here would fail to
> > > detect the actual deadlock on single-threaded workqueues, which do
> >
> > Do you mean it's true even in case having fixed lockdep properly?
> > Could you explain why if so? IMHO, I don't think so.
>
> I'm saying that if lockdep is fixed it should be:
>
> if (wq->saved_max_active == 1 || wq->rescuer) {
> lock_map_acquire(wq->lockdep_map);
> lock_map_acquire(lockdep_map);
> } else {
> lock_map_acquire_read(wq->lockdep_map);
> lock_map_acquire_read(lockdep_map);
> }
>
> or something like that, because for a single-threaded workqueue, the
> following _IS_ a deadlock:
>
> work-n:
> wait_for_completion(C);
>
> work-n+1:
> complete(C);
>
> And that is the only case we now fail to catch.
Thank you for explanation.
> > > +void crossrelease_hist_start(enum xhlock_context_t c, bool force)
> > > {
> > > struct task_struct *cur = current;
> > >
> > > - if (cur->xhlocks) {
> > > - cur->xhlock_idx_hist[c] = cur->xhlock_idx;
> > > - cur->hist_id_save[c] = cur->hist_id;
> > > + if (!cur->xhlocks)
> > > + return;
> > > +
> > > + /*
> > > + * We call this at an invariant point, no current state, no history.
> > > + */
> >
> > This very work-around code _must_ be removed after fixing read-recursive
> > thing in lockdep. I think it would be better to add a tag(comment)
> > saying it.
> >
> > > + if (c == XHLOCK_PROC) {
> > > + /* verified the former, ensure the latter */
> > > + WARN_ON_ONCE(!force && cur->lockdep_depth);
> > > + invalidate_xhlock(&xhlock(cur->xhlock_idx));
> > > }
>
> No, this is not a work around, this is fundamentally so. It's not going
> away. The only thing that should go away is the .force argument.
I meant, this seems to be led from your mis-understanding of
crossrelease_hist_{start, end}().
Uer of force == 1 should not exist or don't have to exist. I am sure you
haven't read my replys. Please read the following at least:
https://lkml.org/lkml/2017/8/24/126
