Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

[Will Deacon <will.deacon@arm.com>]

On Tue, Jul 07, 2015 at 12:17:31PM +0100, Peter Zijlstra wrote:
> On Tue, Jul 07, 2015 at 10:17:11AM +0100, Will Deacon wrote:
> > > > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg
> > > > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire)
> > > > rmode until it hits zero?
> > > 
> > > No, this is how we make the lock fair so that an incoming streams of 
> > > later readers won't block a writer from getting the lock.
> > 
> > But won't those readers effectively see that the lock is held for write
> > (because we set wmode to _QW_LOCKED before the existing reader had drained)
> > and therefore fall down the slow-path and get held up on the spinlock?
> 
> Yes, that's the entire point. Once there's a writer pending, new readers
> should queue too.

Agreed. My point was that we can achieve the same result without
a separate _QW_WAITING flag afaict.

Will