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

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Message-ID: <20190418091109.GY12232@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Apr 2019 11:11:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

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.