linux-kernel - Re: [RFC PATCH] rcu: introduce kfree

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Message-ID: <48D2882B.9060806@colorfullife.com>
Date:	Thu, 18 Sep 2008 18:56:11 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] rcu: introduce kfree_rcu()

Paul E. McKenney wrote:
> On Thu, Sep 18, 2008 at 12:18:28PM +0800, Lai Jiangshan wrote:
>   
>> sometimes a rcu callback is just calling kfree() to free a struct's memory
>> (we say this callback is a trivial callback.).
>> this patch introduce kfree_rcu() to do these things directly, easily.
>>     
>
> Interesting!  Please see questions and comments below.
>
>   
>> There are 4 reasons that we need kfree_rcu():
>>
>> 1) unloadable modules:
>>    a module(rcu callback is defined in this module) using rcu must
>>    call rcu_barrier() when unload. rcu_barrier() will increase
>>    the system's overhead(the more cpus the worse) and
>>    rcu_barrier() is very time-consuming. if all rcu callback defined
>>    in this module are trivial callback, we can just call kfree_rcu()
>>    instead, save a rcu_barrier() when unload.
>>     
Hmm: why is rcu_barrier() sufficient to prevent races?
Offlining a cpu reorders rcu callbacks - rcu_barrier() can return before 
all previous call_rcu() callbacks were called.

--
    Manfred

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