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

[Peter Zijlstra]

On Thu, Nov 24, 2016 at 12:26:57PM +0100, Nicolai Hähnle 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 :/

> In the meantime, I'd appreciate it if patch #1 could be accepted as-is for
> stable updates to <= 4.8. It fixes a real (if rare) bug, and the stampede
> inefficiency isn't a problem in practice at least for GPU applications.

Sorry can't do. We don't do stable patches that don't have anything
upstream.