Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

[Peter Zijlstra]

On Wed, Apr 17, 2019 at 01:51:24PM -0400, Waiman Long wrote:
> On 04/17/2019 10:05 AM, Peter Zijlstra wrote:
> > On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
> >> @@ -650,6 +686,33 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
> >>  	struct rwsem_waiter waiter;
> >>  	DEFINE_WAKE_Q(wake_q);
> >>  
> >> +	if (!rwsem_can_spin_on_owner(sem))
> >> +		goto queue;
> >> +
> >> +	/*
> >> +	 * Undo read bias from down_read() and do optimistic spinning.
> >> +	 */
> >> +	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> >> +	adjustment = 0;
> >> +	if (rwsem_optimistic_spin(sem, false)) {
> >> +		unsigned long flags;
> >> +
> >> +		/*
> >> +		 * Opportunistically wake up other readers in the wait queue.
> >> +		 * It has another chance of wakeup at unlock time.
> >> +		 */
> >> +		if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS) &&
> >> +		    raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
> > why trylock?

> I used trylock to avoid getting stuck in the spinlock while holding a
> read lock on the rwsem.

Is that a real concern? I would think that not waking further readers
would, esp. under high contention, be a bigger deal.