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Message-ID: <48D2882B.9060806@colorfullife.com>
Date: Thu, 18 Sep 2008 18:56:11 +0200
From: Manfred Spraul <manfred@colorfullife.com>
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To: paulmck@linux.vnet.ibm.com
CC: Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Dipankar Sarma <dipankar@in.ibm.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH] rcu: introduce kfree_rcu()
References: <48D1D694.9010802@cn.fujitsu.com> <20080918064406.GC6397@linux.vnet.ibm.com>
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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