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

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Message-ID: <20090319085628.GA6167@in.ibm.com>
Date:	Thu, 19 Mar 2009 14:26:28 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-input@...r.kernel.org, dmitry.torokhov@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

On Wed, Mar 18, 2009 at 09:58:12PM -0700, 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...
> 

The more appropriate question is - why is synchronize_rcu() taking
1 second ? Any idea what the other CPUs are doing at the time
of calling synchronize_rcu() ? What driver is this ? How early
in the boot is this happening ? 

Thanks
Dipankar
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