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

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Date:	Fri, 29 May 2015 13:41:05 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 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.

Basically, strive to avoid "smp_mb()" at all costs. It's insanely
expensive compared to pretty much all other serialization
alternatives.

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

                  Linus
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