Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

[Peter Zijlstra <peterz@infradead.org>]

On Sat, 2010-05-08 at 08:27 +1000, Benjamin Herrenschmidt wrote:
> > Also, to solve this problem when several cpus may spin on the owner,
> > I wonder adaptive spinning is doing the right thing here. We should
> > have only one spinner I think, and the rest should go to sleep as the
> > time to spin on several subsequent owners would be much better gained
> > to do something else (schedule something else or power saving).
> > In fact, that too could deserve some tests.
> 
> Right, the problem is due to the fact that we skip spinning if there's
> already a waiter but we don't know that there is already a spinner so we
> can end up with multiple spinners.
> 
> I don't see a non invasive way to fix that.. we could add a spinner
> counter to the mutex but that sucks a bit. Might still be worthwhile,
> not sure. Peter, what do you reckon ? 

If its a large problem the lock is overly contended and _that_ needs
fixing. I don't at all feel like adding atomic ops to the spin loop to
try and detect this.

As to the 2 jiffy spin timeout, I guess we should add a lockdep warning
for that, because anybody holding a mutex for longer than 2 jiffies and
not sleeping does need fixing anyway.