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.
> > +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.
