Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

Peter Zijlstra
2009-01-14T16:46:39Z

On Mon, 2009-01-12 at 19:32 +0200, Avi Kivity wrote:
> Peter Zijlstra wrote:
> > Spinlocks can use 'pure' MCS locks.
> >
>
> How about this, then. In mutex_lock(), keep wait_lock locked and only
> release it when scheduling out. Waiter spinning naturally follows. If
> spinlocks are cache friendly (are they today?)

(no they're not, Nick's ticket locks still spin on a shared cacheline
IIRC -- the MCS locks mentioned could fix this)

> we inherit that. If
> there is no contention on the mutex, then we don't need to reacquire the
> wait_lock on mutex_unlock() (not that the atomic op is that expensive
> these days).

That might actually work, although we'd have to move the
__mutex_slowpath_needs_to_unlock() branch outside wait_lock otherwise
we'll deadlock :-)

It might be worth trying this if we get serious fairness issues with the
current construct.