Re: [PATCH 1/4] locking/ww_mutex: Fix a deadlock affecting ww_mutexes

[Peter Zijlstra]

On Thu, Nov 24, 2016 at 12:52:25PM +0100, Daniel Vetter wrote:
> On Thu, Nov 24, 2016 at 12:40 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> >> I do believe we can win a bit by keeping the wait list sorted, if we also
> >> make sure that waiters don't add themselves in the first place if they see
> >> that a deadlock situation cannot be avoided.
> >>
> >> I will probably want to extend struct mutex_waiter with ww_mutex-specific
> >> fields to facilitate this (i.e. ctx pointer, perhaps stamp as well to reduce
> >> pointer-chasing). That should be fine since it lives on the stack.
> >
> > Right, shouldn't be a problem I think.
> >
> > The only 'problem' I can see with using that is that its possible to mix
> > ww and !ww waiters through ww_mutex_lock(.ctx = NULL). This makes the
> > list order somewhat tricky.
> >
> > Ideally we'd remove that feature, although I see its actually used quite
> > a bit :/
> 
> I guess we could create a small fake acquire_ctx for single-lock
> paths. That way callers still don't need to deal with having an
> explicit ctx, but we can assume the timestamp (for ensuring fairness)
> is available for all cases. Otherwise there's indeed a problem with
> correctly (well fairly) interleaving ctx and non-ctx lockers I think.

Actually tried that, but we need a ww_class to get a stamp from, and
ww_mutex_lock() doesn't have one of those..