linux-kernel - Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

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Date:   Wed, 30 Oct 2019 15:21:10 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
        juri.lelli@...hat.com, williams@...hat.com, bristot@...hat.com,
        longman@...hat.com, dave@...olabs.net, jack@...e.com
Subject: Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

On 10/29, Peter Zijlstra wrote:
>
> I like that symmetry, but see below ...

...

> > void __percpu_up_read(struct percpu_rw_semaphore *sem)
> > {
> > 	smp_mb();
> >
> > 	__this_cpu_dec(*sem->read_count);
> >
> 	preempt_enable();
> > 	wake_up(&sem->waiters);
> 	preempt_disable()
>
> and this (sadly) means there's a bunch of back-to-back
> preempt_disable()+preempt_enable() calls.

Hmm. Where did these enable+disable come from?

	void __percpu_up_read(struct percpu_rw_semaphore *sem)
	{
		smp_mb();

		__this_cpu_dec(*sem->read_count);

		wake_up(&sem->waiters);
	}

should work just fine?

Oleg.