linux-kernel - Re: [RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce global impact

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Message-ID: <20150530204900.GA17772@redhat.com>
Date:	Sat, 30 May 2015 22:49:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce
	global impact

On 05/29, Linus Torvalds wrote:
>
> On Fri, May 29, 2015 at 1:09 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Doesn't it need mb() before "state = readers_slow" to ensure
> > "release" semantics?
>
> Please don't do any mb() at all. Just do smp_store_release() if
> release semantics is what you want.

Agreed!

> (And try to make sure to pair it with smp_load_acquire() on the other
> side for things to make sense)

Hmm. I think you are right...

__percpu_down_read() lacks another mb() after the "state != BLOCK"
check for the same reason, and we can use smp_load_acquire(state)
instead.

Oleg.

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