Re: [RFC PATCH] rcu: introduce kfree_rcu()

[Manfred Spraul]

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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