linux-kernel - Re: [PATCH v3] locking/rwsem: Exit read lock slowpath if queue empty & no writer

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Date:   Wed, 8 Aug 2018 11:24:01 +0100
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Joe Mario <jmario@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3] locking/rwsem: Exit read lock slowpath if queue empty
 & no writer

Hi Waiman,

On Tue, Aug 07, 2018 at 07:29:49PM -0400, Waiman Long wrote:
> On 07/24/2018 03:10 PM, Waiman Long wrote:
> > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> > index 3064c50..01fcb80 100644
> > --- a/kernel/locking/rwsem-xadd.c
> > +++ b/kernel/locking/rwsem-xadd.c
> > @@ -233,8 +233,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> >  	waiter.type = RWSEM_WAITING_FOR_READ;
> >  
> >  	raw_spin_lock_irq(&sem->wait_lock);
> > -	if (list_empty(&sem->wait_list))
> > +	if (list_empty(&sem->wait_list)) {
> > +		/*
> > +		 * In case the wait queue is empty and the lock isn't owned
> > +		 * by a writer, this reader can exit the slowpath and return
> > +		 * immediately as its RWSEM_ACTIVE_READ_BIAS has already
> > +		 * been set in the count.
> > +		 */
> > +		if (atomic_long_read(&sem->count) >= 0) {
> > +			raw_spin_unlock_irq(&sem->wait_lock);
> > +			return sem;
> > +		}
> >  		adjustment += RWSEM_WAITING_BIAS;
> > +	}
> >  	list_add_tail(&waiter.list, &sem->wait_list);
> >  
> >  	/* we're now waiting on the lock, but no longer actively locking */
> 
> Will this patch be eligible to go into 4.19 or 4.20?

It's probably worth reposting with the Acks you've received, so that Ingo
can easily pick it up into -tip (but it might be a bit close for 4.19 at
this point).

Will