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

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

On Mon, Jul 06, 2015 at 08:49:33PM +0100, Waiman Long wrote:
> On 07/06/2015 02:23 PM, Will Deacon wrote:
> > I've just finished rebasing my arm64 qrwlock stuff, but I think it will
> > conflict with these patches. Do you mind if I post them for review anyway,
> > so we can at least co-ordinate our efforts?
> 
> Yes, sure. I would also like to coordinate my changes with yours to 
> minimize conflict. BTW, I just got 2 tip-bot messages about the commits:
> 
>     locking/qrwlock:  Better optimization for interrupt context readers
>     locking/qrwlock:  Rename functions to queued_*()
> 
> So I need to rebase my patches also.

Yeah, I've been carrying those two on my branch as well, but everything
should rebase cleanly.

> >> ---
> >>   kernel/locking/qrwlock.c |   12 ++++--------
> >>   1 files changed, 4 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> >> index 81bae99..ecd2d19 100644
> >> --- a/kernel/locking/qrwlock.c
> >> +++ b/kernel/locking/qrwlock.c
> >> @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
> >>   	arch_spin_lock(&lock->lock);
> >>
> >>   	/*
> >> -	 * At the head of the wait queue now, wait until the writer state
> >> -	 * goes to 0 and then try to increment the reader count and get
> >> -	 * the lock. It is possible that an incoming writer may steal the
> >> -	 * lock in the interim, so it is necessary to check the writer byte
> >> -	 * to make sure that the write lock isn't taken.
> >> +	 * At the head of the wait queue now, increment the reader count
> >> +	 * and wait until the writer, if it has the lock, has gone away.
> >> +	 * At ths stage, it is not possible for a writer to remain in the
> >> +	 * waiting state (_QW_WAITING). So there won't be any deadlock.
> >>   	 */
> >> -	while (atomic_read(&lock->cnts)&  _QW_WMASK)
> >> -		cpu_relax_lowlatency();
> > 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?

Will