linux-kernel - Re: Question about usage of RCU in the input layer

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Message-ID: <20090319070221.6aca7245@infradead.org>
Date:	Thu, 19 Mar 2009 07:02:21 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...r.kernel.org, dipankar@...ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

On Thu, 19 Mar 2009 00:23:56 -0700
Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:

> Hi Arjan,
> 
> On Wednesday 18 March 2009 21:58:12 Arjan van de Ven wrote:
> > Hi,
> >
> > the input layer does a "synchronize_rcu()" after a
> > list_add_tail_rcu(), which is costing me 1 second of boot time.....
> > And based on my understanding of the RCU concept, you only need to
> > synchronize on delete, not on addition... so I think the
> > synchronize is entirely redundant here...
> 
> 
> It is there to guarantee that once we registered the handle all
> subsequent input events will be delivered through it.


afaik rcu already guarantees that even without a synchronize;
the only reason you would need a synchronize is to guarantee that
people STOPPED using your memory. Or am I now totally misunderstanding
RCU ?
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