On 24.11.2016 12:56, Peter Zijlstra wrote: > 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 at infradead.org> >> 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..
The acquire context needs to be live until the unlock anyway, so this is something that requires modifying the callers of ww_mutex_lock. Those should all have a ww_class available, or something is very wrong :) Nicolai